feat(tool-fs): result-time applied-hunk diffs for write/edit

fs write/edit now emit a result-time contextual-diff tool_call_update
(the applied hunk with ±3 context lines, one hunk per replace_all site),
matching what claude-agent-acp sends and what makes an editor render the
change in place. The call-time snippet diff stays; the result hunk
supersedes it (ACP content-replace).

Mechanism:
- A persisted tool-private `meta` channel: execute may return
  `{ content, meta }`; `meta` (JsonValue) rides on the tool/result event
  and is handed back to presentResult, so the diff reproduces on replay
  (event-sourced). JsonValue is now exported from dsh-session.
- The backend returns raw before/after text (storage facts) on
  FsWriteOutcome/FsEditOutcome; the tool computes the hunk via the npm
  `diff` package's structuredPatch. A create has no before → no result
  diff; a failed/aborted mutation carries no meta.
- ToolResultView gains a DiffResultView; the bridge's result-side switch
  renders it as {type:'diff'} content blocks.

RFC: docs/rfc/implemented/architecture/2026-07-02-result-time-applied-hunk-diffs.md
(justifies the npm `diff` runtime dep over vendoring and the meta channel);
the render-intent-union RFC's Non-goal is updated to record this shipped.
All fs snapshot goldens re-recorded; edit/overwrite gain the contextual
result diff, create/read/policy-reject unchanged in structure.

packages/core/agent-loop/tests/loop.spec.ts

@@ -110,6 +110,32 @@ describe('agent loop', () => {
     expect(types).toContain('tool/result')
   })
 
+  it('threads a tool-attached meta (execute object return) onto the tool/result event', async () => {
+    const adapter = new MockAdapter([
+      toolCallResponse('c1', 'writer', { path: 'a.txt' }, 'writing'),
+      textResponse('done'),
+    ])
+    const ctx = await harness(adapter)
+    // A tool that returns the { content, meta } object form: the loop must
+    // persist `meta` on the tool/result event so a UI reproduces the card on replay.
+    ctx.tools.register(defineTool({
+      name: 'writer',
+      description: 'writes a file',
+      parameters: { path: { type: 'string' } },
+      async execute() {
+        return { content: [{ type: 'text', text: 'ok' }], meta: { diffs: [{ path: 'a.txt', oldText: null, newText: 'x' }] } }
+      },
+    }))
+    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
+
+    send(agent, 'use the tool')
+    await waitForIdle(ctx, agent)
+
+    const toolResult = agent.session.events.find(e => e.type === 'tool/result')
+    expect(toolResult?.type === 'tool/result' && toolResult.data.meta)
+      .toEqual({ diffs: [{ path: 'a.txt', oldText: null, newText: 'x' }] })
+  })
+
   it('passes assembled system prompt and tool schemas into the request', async () => {
     const adapter = new MockAdapter([textResponse('ok')])
     const ctx = await harness(adapter)