feat: MCP {env:VAR} key substitution + coder model/tool-result fixes + docs refactor (v2.7.9)

- MCP secrets: substituteEnvVars recursively resolves {env:NAME} in mcp.json string values from process.env before Zod (opencode-compatible); unset -> '' + boot warning, and invalid-config log names the unset vars (an empty {env:VAR} in a strict url/command field invalidates the whole config)
- data/mcp.json now untracked (.gitignore flips !data/mcp.json -> !data/mcp.example.json); tracked template data/mcp.example.json carries "{env:CONTEXT7_API_KEY}"; .env.example documents the key (9 mcp-config tests)
- Coder fix: message_complete frame model widened string -> string|null (server+web ws-frames parity); dispatcher publishes model: task.model at all 4 external completion points — a null model otherwise fail-closed in publishFrame and dropped the whole frame incl. status:'complete' (regression test)
- Coder fix: claude-sdk mapUserToolResults maps user-message tool_result blocks -> terminal tool_update events (completed/failed w/ output) so tool snapshots resolve instead of spinning forever
- Composer: AgentComposerBar drops §9b resumed/history/new chip + token readout, loses flex-wrap so the row stays one line; CoderPane gains a per-chat localStorage agent-config cache (restores last model on reopen) + threads model into the timeline/chip
- Docs: root CLAUDE.md slimmed (~190 lines), per-app refs split to apps/{coder,server,web}/CLAUDE.md; new docs/coder-backends.md, docs/project-discovery.md, docs/coding-standards/ (cross-app-contract-parity); ARCHITECTURE.md links the backends doc

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/coder/src/services/backends/__tests__/claude-sdk-map.test.ts

@@ -179,3 +179,73 @@ describe('mapSdkMessage — non-content messages', () => {
     ).toEqual([]);
   });
 });
+
+describe('mapSdkMessage — user tool results', () => {
+  /** A `user` message carrying tool_result blocks (the SDK feeds tool output back here). */
+  function userMsg(content: unknown): SDKMessage {
+    return msg({ type: 'user', message: { role: 'user', content }, parent_tool_use_id: null, uuid: 'u', session_id: 's' });
+  }
+
+  it('maps a string tool_result to a completed tool_update carrying the output', () => {
+    const state = createClaudeSdkMapState();
+    const out = mapSdkMessage(userMsg([{ type: 'tool_result', tool_use_id: 't1', content: 'done' }]), state);
+    expect(out).toEqual<AgentEvent[]>([
+      {
+        type: 'tool_update',
+        toolCall: { toolCallId: 't1', title: 't1', kind: null, status: 'completed', rawInput: undefined, rawOutput: 'done' },
+      },
+    ]);
+  });
+
+  it('marks an is_error result failed', () => {
+    const state = createClaudeSdkMapState();
+    const out = mapSdkMessage(userMsg([{ type: 'tool_result', tool_use_id: 't1', content: 'boom', is_error: true }]), state);
+    const ev = out[0]!;
+    if (ev.type !== 'tool_update') throw new Error('expected tool_update');
+    expect(ev.toolCall.status).toBe('failed');
+    expect(ev.toolCall.rawOutput).toBe('boom');
+  });
+
+  it('flattens array text blocks (skipping non-text) and reuses a prior snapshot title', () => {
+    const state = createClaudeSdkMapState();
+    mapSdkMessage(
+      streamEvent({ type: 'content_block_start', index: 1, content_block: { type: 'tool_use', id: 't2', name: 'view_file', input: {} } }),
+      state,
+    );
+    const out = mapSdkMessage(
+      userMsg([
+        {
+          type: 'tool_result',
+          tool_use_id: 't2',
+          content: [
+            { type: 'text', text: 'line1' },
+            { type: 'image', source: {} },
+            { type: 'text', text: 'line2' },
+          ],
+        },
+      ]),
+      state,
+    );
+    const ev = out[0]!;
+    if (ev.type !== 'tool_update') throw new Error('expected tool_update');
+    expect(ev.toolCall.toolCallId).toBe('t2');
+    expect(ev.toolCall.title).toBe('view_file');
+    expect(ev.toolCall.status).toBe('completed');
+    expect(ev.toolCall.rawOutput).toBe('line1\nline2');
+  });
+
+  it('surfaces a result for an unknown tool_use_id with the id as the title', () => {
+    const state = createClaudeSdkMapState();
+    const out = mapSdkMessage(userMsg([{ type: 'tool_result', tool_use_id: 'orphan-id', content: 'x' }]), state);
+    expect(out[0]).toMatchObject({
+      type: 'tool_update',
+      toolCall: { toolCallId: 'orphan-id', title: 'orphan-id', kind: null, status: 'completed' },
+    });
+  });
+
+  it('ignores non-tool_result blocks and non-array content', () => {
+    const state = createClaudeSdkMapState();
+    expect(mapSdkMessage(userMsg([{ type: 'text', text: 'hi' }]), state)).toEqual([]);
+    expect(mapSdkMessage(userMsg('plain string'), state)).toEqual([]);
+  });
+});