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/web/src/components/panes/CoderPane.tsx

@@ -30,6 +30,8 @@ interface CoderMessage {
   role: 'user' | 'assistant' | 'system';
   content: string;
   status?: 'streaming' | 'complete' | 'failed';
+  // model-attribution: which model produced this assistant message (chip).
+  model?: string | null;
   reasoning_text?: string;
   tool_calls?: Array<{
     id: string;
@@ -52,6 +54,46 @@ interface CoderToolMessage {
 
 type CoderTimelineMessage = CoderMessage | CoderToolMessage;
 
+// Per-chat agent-config cache (provider/model/mode/thinking). Keyed by chat id
+// so reopening or switching back to a chat restores the model that was loaded
+// last there. Per-device (localStorage) — a UI convenience, not authoritative.
+const DEFAULT_AGENT_CONFIG: AgentSessionConfig = {
+  provider: 'boocode',
+  model: '',
+  modeId: null,
+  thinkingOptionId: null,
+};
+function agentConfigKey(chatId: string | undefined): string | null {
+  return chatId ? `boocode.coder.config.${chatId}` : null;
+}
+function readCachedAgentConfig(chatId: string | undefined): AgentSessionConfig | null {
+  const key = agentConfigKey(chatId);
+  if (!key || typeof localStorage === 'undefined') return null;
+  try {
+    const raw = localStorage.getItem(key);
+    if (!raw) return null;
+    const c = JSON.parse(raw) as Partial<AgentSessionConfig>;
+    if (typeof c?.provider !== 'string') return null;
+    return {
+      provider: c.provider,
+      model: typeof c.model === 'string' ? c.model : '',
+      modeId: c.modeId ?? null,
+      thinkingOptionId: c.thinkingOptionId ?? null,
+    };
+  } catch {
+    return null;
+  }
+}
+function writeCachedAgentConfig(chatId: string | undefined, config: AgentSessionConfig): void {
+  const key = agentConfigKey(chatId);
+  if (!key || typeof localStorage === 'undefined') return;
+  try {
+    localStorage.setItem(key, JSON.stringify(config));
+  } catch {
+    /* quota / disabled storage — non-fatal */
+  }
+}
+
 interface PendingChange {
   id: string;
   file_path: string;
@@ -97,6 +139,7 @@ type RawCoderMessage = {
   chat_id?: string;
   content?: string | null;
   status?: string | null;
+  model?: string | null;
   reasoning_text?: string;
   reasoning_parts?: Array<{ text?: string }> | null;
   tool_results?: {
@@ -144,6 +187,7 @@ function mapCoderTimelineRow(raw: RawCoderMessage): CoderTimelineMessage | null
     role: raw.role as CoderMessage['role'],
     content: raw.content ?? '',
     status: (raw.status ?? 'complete') as CoderMessage['status'],
+    ...(raw.model ? { model: raw.model } : {}),
     ...(reasoning_text ? { reasoning_text } : {}),
     ...(tool_calls?.length ? { tool_calls } : {}),
     ctx_used: raw.ctx_used ?? null,
@@ -253,6 +297,7 @@ function useCoderMessages(sessionId: string, chatId: string | undefined, handler
                 ? {
                     ...m,
                     status: 'complete' as const,
+                    model: (frame as any).model ?? (m as any).model ?? null,
                     ctx_used: (frame as any).ctx_used ?? (m as any).ctx_used ?? null,
                     ctx_max: (frame as any).ctx_max ?? (m as any).ctx_max ?? null,
                   }
@@ -586,12 +631,37 @@ export function CoderPane({
   onConnectedChange,
   onAgentLabelChange,
 }: Props) {
-  const [agentConfig, setAgentConfig] = useState<AgentSessionConfig>({
-    provider: 'boocode',
-    model: '',
-    modeId: null,
-    thinkingOptionId: null,
-  });
+  const [agentConfig, setAgentConfig] = useState<AgentSessionConfig>(
+    () => readCachedAgentConfig(chatId) ?? DEFAULT_AGENT_CONFIG,
+  );
+  // Restore the per-chat cached config when the chat changes. The ref guard
+  // skips the initial mount (lazy init already loaded it) + StrictMode double-runs.
+  const lastLoadedChatRef = useRef<string | undefined>(chatId);
+  useEffect(() => {
+    const prev = lastLoadedChatRef.current;
+    if (prev === chatId) return;
+    lastLoadedChatRef.current = chatId;
+    // undefined → real id: the pane just resolved its chat. A selection made
+    // while chatId was undefined could not be persisted (the key was null), so
+    // carry the current in-memory config into the new chat — and persist it —
+    // rather than clobbering the user's pick with DEFAULT on the cache miss.
+    if (prev === undefined && chatId) {
+      const cached = readCachedAgentConfig(chatId);
+      if (cached) setAgentConfig(cached);
+      else writeCachedAgentConfig(chatId, agentConfig);
+      return;
+    }
+    setAgentConfig(readCachedAgentConfig(chatId) ?? DEFAULT_AGENT_CONFIG);
+  }, [chatId, agentConfig]);
+  // Persist on user-driven changes only (not on the restore above), so switching
+  // chats never clobbers the new chat's cached config with the old one.
+  const handleAgentConfigChange = useCallback(
+    (next: AgentSessionConfig) => {
+      setAgentConfig(next);
+      writeCachedAgentConfig(chatId, next);
+    },
+    [chatId],
+  );
 
   useEffect(() => {
     const parts = [agentConfig.provider || 'boocode'];
@@ -727,13 +797,6 @@ export function CoderPane({
     }
   }, [messages, refresh, refreshCheckpoints, sessionId]);
 
-  // The §9b chip only shows once the chat has ≥1 prior turn (a completed
-  // assistant message). Hidden on a brand-new chat.
-  const hasPriorTurn = useMemo(
-    () => messages.some((m) => m.role === 'assistant' && (m as CoderMessage).status === 'complete'),
-    [messages],
-  );
-
   // Poll fallbacks when WS is disconnected (reconnect uses WS as source of truth)
   useEffect(() => {
     if (!activeTaskId || connected) return;
@@ -1001,11 +1064,9 @@ export function CoderPane({
       <AgentComposerBar
         projectPath={projectPath}
         value={agentConfig}
-        onChange={setAgentConfig}
+        onChange={handleAgentConfigChange}
         onProviderCommandsChange={handleProviderCommandsChange}
         connected={connected}
-        sessionId={sessionId}
-        hasPriorTurn={hasPriorTurn}
         agentStatus={currentAgentStatus}
       />
       {/* Chat area — BooChat-style timeline (text + tool runs as siblings) */}