refactor(dev-infra): narrow worktree hook safety checks

.agents/notes/implemented/process/2026-07-27-worktree-local-lefthook.md

@@ -12,13 +12,13 @@ Lefthook-generated hooks prefer an absolute binary path captured from the instal
 
 ## Decision
 
-Hook installation is worktree-scoped. With `CI=true` or `GITHUB_ACTIONS=true`, the installer returns before Git discovery or mutation because automated jobs do not consume contributor hooks. Otherwise, it requires Git 2.26 or newer for configuration-scope provenance, upgrades a format-0 repository to format 1, enables `extensions.worktreeConfig`, and assigns the current worktree an absolute `core.hooksPath` at `$GIT_DIR/dsh-hooks`. Before the format bump, it refuses every existing `extensions.*` key in the common config or a conditional target because format 1 would activate that dormant repository extension. Before first enabling the worktree-config extension, it inspects the `config.worktree` file for the main worktree and every registered linked worktree and refuses dormant settings whose activation would change the current or a sibling worktree. The common repository config and every active or dormant worktree config must be regular files. The main worktree receives `$GIT_COMMON_DIR/dsh-hooks`; each linked worktree receives the corresponding directory under `$GIT_COMMON_DIR/worktrees/<id>`. A repository-scoped lock serializes configuration migration and hook writes, including repeated concurrent installs. Each lock records a process ID and random ownership token; release verifies the same file identity and exact record. A dead or invalid lock is never broken automatically, so the diagnostic requires the contributor to confirm no installer is running and remove the lock manually.
+Hook installation is worktree-scoped. With `CI=true` or `GITHUB_ACTIONS=true`, the installer returns before Git discovery or mutation because automated jobs do not consume contributor hooks. Otherwise, it requires Git 2.26 or newer for configuration-scope provenance, upgrades a format-0 repository to format 1, enables `extensions.worktreeConfig`, and assigns the current worktree an absolute `core.hooksPath` at `$GIT_DIR/dsh-hooks`.
 
-The installer recognizes its hook directory with a private ownership marker and updates it idempotently. The marker records the absolute path last published to worktree config, so moving a checkout permits the installer to replace that exact stale owned value with the moved `$GIT_DIR/dsh-hooks` path and regenerate hooks; any other worktree-scoped value remains user-owned and is refused. Before invoking Lefthook, the marker and every existing generated hook must be an unaliased regular file, preventing a symlink or additional hard link from redirecting an overwrite outside the owned directory. It inspects the effective scope, origin, and value of `core.hooksPath`, then refuses an unowned directory, every command-scoped path, and every non-owned worktree-scoped path, including values loaded through `config.worktree` includes. It follows conditional includes with Git's parser and refuses a command- or worktree-scoped include whose target provides, or cannot safely be shown not to provide, a hook path; an inactive condition therefore cannot later hide a user-owned path behind the installer's direct value. The same risk in an inherited system, global, or common-repository include requires `DSH_LEFTHOOK_ALLOW_HOOKS_PATH_OVERRIDE=1`, which explicitly opts only the current worktree into Lefthook while other worktrees retain the inherited path. Unrelated conditional includes remain valid. Command-scoped Git configuration is removed from the Lefthook subprocess environment after validation. This opt-in does not attempt to chain arbitrary hook managers.
+Before upgrading format 0, the installer refuses direct common-config `extensions.*`; it also refuses direct `core.worktree` or `core.bare=true` and non-empty dormant worktree configs that enabling the extension would activate. The migration removes direct `core.bare=false` because false is Git's default. The common repository config and every existing `config.worktree` must be regular files. These checks disable include expansion because Git's repository-format parser also ignores included targets. A repository-scoped lock serializes migration and hook writes; its process ID, random token, file identity, and exact contents must still match at release. Dead or invalid locks require manual recovery rather than automatic breaking.
 
-Enabling worktree config removes the standard redundant `core.bare=false` value from the common config because false remains Git's default; an explicit `core.worktree` or `core.bare=true`, whether direct or loaded through an active common-config include, is refused for manual migration. Before enabling the extension, the installer follows common-config conditional includes and refuses a target that provides, or cannot safely be shown not to provide, either migration-sensitive key; unrelated conditional includes remain valid. If Lefthook fails during a first install, the installer removes the new worktree override so the prior inherited or common hooks remain active. If that rollback also fails, one diagnostic preserves both failures for manual recovery. Legacy files in `$GIT_COMMON_DIR/hooks` are never removed or rewritten by the worktree-local installer.
+Each hook directory carries a JSON ownership marker containing the absolute path last published to worktree config. After a checkout moves, that marker permits replacement of only the exact stale owned value. Before Lefthook runs, the marker and every existing generated hook must be unaliased regular files. The installer resolves the effective scope, origin, and value of `core.hooksPath`, including active `config.worktree` includes; it refuses command-scoped paths, unowned worktree-scoped paths, and unowned reserved directories. An inherited system, global, or common-repository path requires `DSH_LEFTHOOK_ALLOW_HOOKS_PATH_OVERRIDE=1`, which opts only the current worktree into Lefthook. Inactive `includeIf` targets are not recursively inspected because they do not affect the current configuration. Command-scoped Git configuration is removed from the Lefthook subprocess environment after validation.
 
-[`install-lefthook.spec.ts`](../../../../scripts/install-lefthook.spec.ts) exercises the CI no-op, main and linked worktrees, removal independence, repeated and concurrent installs, checkout relocation, marker and hook alias refusal, stale and replaced lock ownership, the Git version boundary, dormant repository-extension and sibling-config refusal, common and worktree config file ownership, migration keys loaded through active and conditional common-config includes, scoped custom-path refusal and opt-in, active and inactive worktree includes, inherited conditional paths, command-environment isolation, legacy common-hook preservation, and successful and failed rollback after installation failure.
+If Lefthook fails after changing `core.hooksPath`, the installer restores the previous worktree value; a rollback failure is reported alongside the installation failure. Existing files in `$GIT_COMMON_DIR/hooks` are never removed or rewritten. Focused installer tests pin isolation, migration refusal, ownership and relocation, concurrent installation, custom paths, and rollback.
 
 ## Alternatives considered
 
@@ -36,6 +36,6 @@ Enabling worktree config removes the standard redundant `core.bare=false` value
 
 Installing or removing one worktree no longer changes another worktree's active hooks, binary path, or generated hook bytes. Concurrent installs are serialized and repeated installation is idempotent, while the jobs and latency boundary owned by [Fast local Git hooks](2026-07-22-fast-local-git-hooks.md) stay unchanged.
 
-The repository becomes a Git format-1 repository after the first installation and rejects clients older than Git 2.26. Custom worktree hook managers require an explicit integration choice; inherited hook paths can coexist across other worktrees, but opting the current worktree into Lefthook means those inherited hooks do not run there unless the contributor chains them through `lefthook.yml`.
+The repository becomes a Git format-1 repository after the first installation. The installer requires Git 2.26 for `--show-scope`; the worktree-config extension itself predates that command. Custom worktree hook managers require an explicit integration choice; inherited hook paths can coexist across other worktrees, but opting the current worktree into Lefthook means those inherited hooks do not run there unless the contributor chains them through `lefthook.yml`.
 
 Legacy common hooks remain on disk for unupgraded worktrees. They can become stale, but removing them automatically would break a registered worktree whose branch has not adopted this installer.