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/fs-write/session.jsonl

@@ -62,7 +62,7 @@
 {"type":"assistant/chunk","seq":60,"time":1783352079886,"data":{"turn":1,"step":1,"chunk":{"type":"finish","reason":{"kind":"tool-calls"}}}}
 {"type":"assistant/message","seq":61,"time":1783352079888,"data":{"turn":1,"step":1,"content":[{"type":"reasoning","text":"The user wants me to create a file named notes.txt with the content \"hello world\" using the write tool, then reply with \"DONE\"."},{"type":"tool-call","id":"call_00_APMUCJJm9lrTSlVbg6dB0185","name":"write","arguments":"{\"file_path\": \"notes.txt\", \"content\": \"hello world\"}"}],"usage":{"inputTokens":2891,"outputTokens":92,"cacheReadTokens":0,"reasoningTokens":30}},"sourceEventSeqs":[4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60],"surfaceOp":"append"}
 {"type":"tool/call","seq":62,"time":1783352079888,"data":{"turn":1,"step":1,"callId":"call_00_APMUCJJm9lrTSlVbg6dB0185","name":"write","arguments":"{\"file_path\": \"notes.txt\", \"content\": \"hello world\"}"}}
-{"type":"tool/result","seq":63,"time":1783352079897,"data":{"turn":1,"step":1,"callId":"call_00_APMUCJJm9lrTSlVbg6dB0185","content":[{"type":"text","text":"Error: unknown tool \"write\""}],"isError":true,"error":{"name":"ToolNotFoundError","code":"UNKNOWN_TOOL"}},"sourceEventSeqs":[62],"surfaceOp":"append"}
+{"type":"tool/result","seq":63,"time":1783352079897,"data":{"turn":1,"step":1,"callId":"call_00_APMUCJJm9lrTSlVbg6dB0185","content":[{"type":"text","text":"<path>/tmp/acp-snap-cwd-sNvn5N/notes.txt</path>\n<type>file</type>\n<content>\nCreated file\n</content>"}],"isError":false},"sourceEventSeqs":[62],"surfaceOp":"append"}
 {"type":"step/end","seq":64,"time":1783352079898,"data":{"turn":1,"step":1}}
 {"type":"step/start","seq":65,"time":1783352079899,"data":{"turn":1,"step":2}}
 {"type":"assistant/chunk","seq":66,"time":1783352080825,"data":{"turn":1,"step":2,"chunk":{"type":"block-start","index":0,"blockType":"reasoning"}}}

examples/acp-agent/tests/snapshots/fs-write/stdout.golden.jsonl

@@ -30,8 +30,8 @@
 {"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"{{sessionId}}","update":{"sessionUpdate":"agent_thought_chunk","content":{"type":"text","text":"D"}}}}
 {"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"{{sessionId}}","update":{"sessionUpdate":"agent_thought_chunk","content":{"type":"text","text":"ONE"}}}}
 {"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"{{sessionId}}","update":{"sessionUpdate":"agent_thought_chunk","content":{"type":"text","text":"\"."}}}}
-{"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"{{sessionId}}","update":{"sessionUpdate":"tool_call","toolCallId":"call_00_APMUCJJm9lrTSlVbg6dB0185","title":"write","kind":"other","status":"in_progress","rawInput":{"file_path":"notes.txt","content":"hello world"}}}}
-{"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"{{sessionId}}","update":{"sessionUpdate":"tool_call_update","toolCallId":"call_00_APMUCJJm9lrTSlVbg6dB0185","status":"failed","content":[{"type":"content","content":{"type":"text","text":"Error: unknown tool \"write\""}}]}}}
+{"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"{{sessionId}}","update":{"sessionUpdate":"tool_call","toolCallId":"call_00_APMUCJJm9lrTSlVbg6dB0185","title":"Write notes.txt","kind":"edit","status":"in_progress","locations":[{"path":"notes.txt"}],"content":[{"type":"diff","path":"notes.txt","oldText":null,"newText":"hello world"}]}}}
+{"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"{{sessionId}}","update":{"sessionUpdate":"tool_call_update","toolCallId":"call_00_APMUCJJm9lrTSlVbg6dB0185","status":"completed","content":[{"type":"diff","path":"notes.txt","oldText":null,"newText":"hello world"}],"title":"Write notes.txt"}}}
 {"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"{{sessionId}}","update":{"sessionUpdate":"agent_thought_chunk","content":{"type":"text","text":"The"}}}}
 {"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"{{sessionId}}","update":{"sessionUpdate":"agent_thought_chunk","content":{"type":"text","text":" file"}}}}
 {"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"{{sessionId}}","update":{"sessionUpdate":"agent_thought_chunk","content":{"type":"text","text":" has"}}}}