feat(sandbox): cross-family file sandbox — one policy home, sandboxed fs provider, fs escalation parity

Extend SandboxMode enforcement from bash to the filesystem tools, the sandbox
RFC's deferred cross-family phase.

- dsh-sandbox-policy (new, ctx.sandboxPolicy): the single home for the
  deployment default mode + workspaceRoot and the per-session override event,
  renamed bash/sandbox-mode -> sandbox/mode and moved here with its fold/setter.
  Decouples the bash seam from dsh-session.
- dsh-fs-sandbox (new): SandboxedFileSystem extends LocalFileSystem and fences
  write/edit by the per-call mode (read-only denies, workspace-write contains to
  the workspace + temp roots via the shared writableRoots, danger passes
  through); reads pass through. Structured FS_SANDBOX_DENIED; in-lock parent
  re-canonicalization. A policy fence in trusted code, not a kernel boundary.
- dsh-sandbox: the shared escalation kit (writableRoots, the strictly-wider
  ladder, denial/hint markers, approveEscalation) both tool families use;
  approveEscalation takes a structural approver so dsh-sandbox gains no
  approval/agent dependency, and both tools stay duplication-free.
- tool-fs: write/edit advertise sandbox_permissions/justification under a
  confining ctx.fs, map FS_SANDBOX_DENIED to the shared [sandbox: ...] marker,
  and resolve the same one-approved-wider retry.
- examples/acp-agent: composes sandbox-policy + fs-sandbox, drops the gating
  that disabled the fs stack under confined modes.

RFC docs/rfc/implemented/feature/2026-07-14-cross-family-fs-sandbox.md; the old
sandbox RFC's In-process/deferred/FAQ sections updated to shipped fact.

examples/acp-agent/composition.md

@@ -12,6 +12,8 @@ flowchart LR
   cfg --> plugin_acp_llm_deepseek
   plugin_acp_sandbox["sandbox<br/>@deepseek-ai/dsh-sandbox-local"]
   cfg --> plugin_acp_sandbox
+  plugin_acp_sandbox_policy["sandbox-policy<br/>@deepseek-ai/dsh-sandbox-policy"]
+  cfg --> plugin_acp_sandbox_policy
   plugin_acp_bash["bash<br/>@deepseek-ai/dsh-bash-sandbox"]
   cfg --> plugin_acp_bash
   plugin_acp_approval["approval<br/>@deepseek-ai/dsh-user-approval"]
@@ -45,8 +47,8 @@ flowchart LR
   cfg --> plugin_acp_tool_todo
   plugin_acp_repeat_tool_guard["repeat-tool-guard<br/>@deepseek-ai/dsh-repeat-tool-guard"]
   cfg --> plugin_acp_repeat_tool_guard
-  plugin_acp_fs_local["fs-local<br/>@deepseek-ai/dsh-fs-local"]
-  cfg --> plugin_acp_fs_local
+  plugin_acp_fs_sandbox["fs-sandbox<br/>@deepseek-ai/dsh-fs-sandbox"]
+  cfg --> plugin_acp_fs_sandbox
   plugin_acp_fs_policy["fs-policy<br/>@deepseek-ai/dsh-fs-policy"]
   cfg --> plugin_acp_fs_policy
   plugin_acp_tool_fs["tool-fs<br/>@deepseek-ai/dsh-tool-fs"]
@@ -61,6 +63,7 @@ flowchart LR
 | --- | --- |
 | `llm-deepseek` | `@deepseek-ai/dsh-llm-deepseek` |
 | `sandbox` | `@deepseek-ai/dsh-sandbox-local` |
+| `sandbox-policy` | `@deepseek-ai/dsh-sandbox-policy` |
 | `bash` | `@deepseek-ai/dsh-bash-sandbox` |
 | `approval` | `@deepseek-ai/dsh-user-approval` |
 | `permission` | `@deepseek-ai/dsh-permission` |
@@ -74,7 +77,7 @@ flowchart LR
 | `tool-workflow` | `@deepseek-ai/dsh-tool-workflow` |
 | `tool-todo` | `@deepseek-ai/dsh-tool-todo` |
 | `repeat-tool-guard` | `@deepseek-ai/dsh-repeat-tool-guard` |
-| `fs-local` | `@deepseek-ai/dsh-fs-local` |
+| `fs-sandbox` | `@deepseek-ai/dsh-fs-sandbox` |
 | `fs-policy` | `@deepseek-ai/dsh-fs-policy` |
 | `tool-fs` | `@deepseek-ai/dsh-tool-fs` |
 | `hooks-claude` | `@deepseek-ai/dsh-hooks-claude` |