feat(sandbox): deny confined executions read access to the credential document

The credential store is 0600 under a 0700 directory, which stops other OS
users but not the model: tool processes run as the same user, so under
the shipped danger-full-access default they read it like any other file.

SandboxExecutionPolicy grows readDenyPaths, and sandbox-policy defaults
it to $DSH_HOME/.env — the exact file rather than the harness home, so
the model keeps its documented access to its own session log. Seatbelt
appends a trailing deny (last matching rule wins) and bwrap maps
/dev/null over each path after any workspace bind; Landlock grants are a
pure allow-list that cannot subtract from its own / read grant, so
confine() reports partial enforcement there instead of claiming a
boundary the process does not have.

A real-kernel Seatbelt e2e proves the shape: the same read succeeds
unconfined and fails under the denial, while a sibling file in the same
directory stays readable. Both READMEs state the residual boundary
plainly — no confining mode means no boundary — and record the OS
keychain provider as the real answer.

packages/credentials/credentials-local/README.md

@@ -22,7 +22,7 @@ The environment wins because a launch-time override (`DEEPSEEK_API_KEY=… dsh`,
 
 ## The document
 
-dotenv format, parsed with `dotenv` and edited by a line editor that preserves every byte it does not own: `set` rewrites the first assignment of its key in place (dropping later duplicates, which dotenv's last-wins reading would otherwise let override the edit), `unset` removes only the owning line, comments and unrelated lines survive verbatim. Writes go through [`dsh-atomic-write`](../../util/atomic-write/README.md) with mode `0600`.
+dotenv format, parsed with `dotenv` and edited by a physical-line editor that preserves every byte it does not own: `set` rewrites the first assignment of its key in place with that line's own ending (dropping later duplicates, which dotenv's last-wins reading would otherwise let override the edit), `unset` removes only the owning line, and comments, unrelated lines, CRLF endings, and the continuation lines of another key's quoted multi-line value all survive verbatim. Every write first re-reads the document under the cross-process writer lock of [`dsh-atomic-write`](../../util/atomic-write/README.md) and publishes anything it had not observed, then commits atomically with mode `0600` under an owner-only (`0700`) directory — so a concurrent writer or an external edit inside the watcher's debounce window is folded in rather than overwritten.
 
 Values are rendered in the narrowest style dotenv reads back verbatim — bare, then single-quoted (fully literal), then double-quoted (only without backslashes, which double-quote reading expands). A value no style can represent, and any entry that already spans multiple physical lines, fails loud instead of being corrupted silently. An empty stored value is absent, per the seam rule.
 
@@ -30,6 +30,15 @@ Values are rendered in the narrowest style dotenv reads back verbatim — bare,
 
 External edits publish `credentials/updated` per changed reference after the snapshot is replaced **wholesale** — an entry deleted on disk never lingers in memory. The provider's own writes are recognized by content and publish exactly their one commit event. An unreadable document at runtime keeps the last good snapshot and warns; an absent file is an empty store; an unreadable file at boot fails loud. Keys that are not POSIX identifiers are preserved file content the seam cannot address.
 
+## Security boundary
+
+The document is `0600` under a `0700` directory, which stops other OS users — **not** the model. Tool processes (bash, the filesystem tools) run as the same user, so under the shipped `danger-full-access` default they can read this file exactly like any other file the user owns. Two things narrow that:
+
+- A **confining sandbox mode** denies the credential document specifically: [`dsh-sandbox-policy`](../../sandbox/sandbox-policy/README.md) defaults `readDenyPaths` to `$DSH_HOME/.env`, and the Seatbelt and bwrap backends enforce it (Landlock cannot subtract from its own `/` read grant and reports `partial`). The denial names the file, not the home, so the model keeps its documented access to its own session log.
+- The harness never hands the model a resolved path to the document, and never loads it into the process environment (see [app-boot's Personal config](../../ui/app-boot/README.md#personal-config)).
+
+Neither makes an unconfined agent safe. A deployment that must keep provider keys away from its own agent should run a confining mode; an OS-keychain provider — a store the model's processes cannot read at all — is the deferred answer and belongs beside this provider as a sibling package.
+
 ## Model Experience
 
 Indirectly, through the consuming LLM adapters: stored values authorize their provider requests, and the adapter owns every model-visible surface.
@@ -40,7 +49,9 @@ No direct invalidation; credentials never enter a request prefix.
 
 ## Known Limitations and Deferred Work
 
-- **Multi-line entries refuse `set`/`unset`** — the line editor will not rewrite an entry it would corrupt; edit the file directly.
+- **Multi-line entries refuse `set`/`unset`** — the line editor will not rewrite an entry it would corrupt; `describe` reports them `writable: false` and edits must go to the file directly.
+- **Same-reference concurrent writes are last-write-wins** — the writer lock and the read-modify-write keep concurrent writers from dropping each other's entries, but two writers editing one reference still resolve to the later write; there is no revision check.
+- **A same-UID process can read the document** — see [Security boundary](#security-boundary): only a confining sandbox mode denies it, and an OS-keychain provider is deferred.
 - **Unrepresentable values fail loud** — control characters, or a mix of both quote styles with backslashes, cannot round-trip the dotenv line format.
 - **Environment changes are invisible** — `process.env` is read live per resolution, but no event can announce a change there.
 - **Atomic, not crash-durable** — inherited from `dsh-atomic-write`; the store re-reads on boot.