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/tests/snapshots/code-mode-turn/system-prompt.golden.md

@@ -5,6 +5,12 @@ You are a coding assistant powered by the deepseek-v4-flash model. Your working
 Verify your work by running the code or tests. Keep answers brief and factual.
 
 
+Use the read tool — not shell commands like cat — to inspect text files. Results include line numbers. Use offset and limit to continue reading large files.
+
+Use the write tool to create files or completely replace file contents. Existing files are overwritten, so read an existing file first (the default fs-policy requires it) and prefer edit for targeted changes.
+
+Use the edit tool for targeted changes to existing UTF-8 text files. It replaces literal old_string with new_string; by default old_string must appear exactly once. If old_string appears multiple times, provide a more specific old_string or set replace_all to true. Read the file first (the default fs-policy requires it), unless you just created or edited it in this session.
+
 Check the [exit code: N] marker on every bash result; investigate failures before moving on.
 
 Approval prompts are disabled in this session: actions that require approval are rejected automatically — do not request sandbox escalation (do not set `sandbox_permissions`).
@@ -52,6 +58,30 @@ declare const tools: {
     /** Task id returned by the bash tool. */
     task_id: string;
   }): Promise<string>;
+  /** Edit an existing UTF-8 text file by replacing literal text. */
+  edit(args: {
+    /** Path to edit, resolved by the filesystem backend. */
+    file_path: string;
+    /** Literal text to replace. Must match exactly. */
+    old_string: string;
+    /** Literal replacement text. Use an empty string to delete the match. */
+    new_string: string;
+    /** Replace all matches. Defaults to false; when false, old_string must appear exactly once. */
+    replace_all?: boolean;
+    /** The wider sandbox mode this file operation needs. Only valid as a one-shot retry of an operation the sandbox just denied; requires justification and user approval. */
+    sandbox_permissions?: "workspace-write" | "danger-full-access";
+    /** Required with sandbox_permissions: one sentence for the user explaining why this exact file operation needs the wider access. */
+    justification?: string;
+  }): Promise<string>;
+  /** Read a UTF-8 text file and return line-numbered content. */
+  read(args: {
+    /** Path to read, resolved by the filesystem backend. */
+    file_path: string;
+    /** 1-based first line to return. Defaults to 1. */
+    offset?: number;
+    /** Maximum number of lines to return. Defaults to 2000. */
+    limit?: number;
+  }): Promise<string>;
   /** Load the full instructions for an available skill. Call this with the exact skill name from the session skill catalog before acting on a task that names or clearly matches that skill. */
   skill(args: {
     /** The exact skill name from the available skills list. */
@@ -106,5 +136,16 @@ declare const tools: {
     /** Optional JSON input exposed to the script as the `args` global (wrap a bare list as a field, e.g. {"files": [...]}). */
     args?: Record<string, unknown>;
   }): Promise<string>;
+  /** Create or fully replace a UTF-8 text file. */
+  write(args: {
+    /** Path to write, resolved by the filesystem backend. */
+    file_path: string;
+    /** Full UTF-8 text content to write. */
+    content: string;
+    /** The wider sandbox mode this file operation needs. Only valid as a one-shot retry of an operation the sandbox just denied; requires justification and user approval. */
+    sandbox_permissions?: "workspace-write" | "danger-full-access";
+    /** Required with sandbox_permissions: one sentence for the user explaining why this exact file operation needs the wider access. */
+    justification?: string;
+  }): Promise<string>;
 }
 ```