feat(mcp): sync MCP servers across enabled CLIs

Adds capabilities.mcpConfig to the CLI registry (Claude, Gemini, Codex,
OpenCode), an additive src/mcp-sync.ts, GET/POST /api/mcp-sync and a
Settings > Agents & CLIs control. Never edits or removes an existing
server; backs up each file it changes; reports conflicts.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/config/cli-registry/stock.ts

@@ -306,6 +306,7 @@ const CLAUDE: CliEntry = {
       'CLAUDE_CONFIG_DIR',
     ],
     gates: { nameFlag: { minVersion: '2.1.224', failClosed: true } },
+    mcpConfig: { path: '.claude.json', format: 'claude-json' },
     // Custom Model Endpoint Profiles (docs/custom-model-endpoints-plan.md) — verified by hand against a real
     // llama.cpp server. Claude reads these at process start only, so switching requires a
     // respawn, never a live hot-swap.
@@ -486,6 +487,7 @@ const OPENCODE: CliEntry = {
     ...agentDefaults(),
     altScreen: 'strip-mux-only',
     echo: { policy: 'buffer', anchor: { kind: 'cursor' }, predictProfile: undefined },
+    mcpConfig: { path: '.config/opencode/opencode.json', format: 'opencode-json' },
     // Verified by hand against a real llama.cpp server. Reuses the SAME env var opencode's
     // own `env.configContentVar` already declares — the builder in custom-model-injection.ts
     // must merge into whatever opencode config Codeman would otherwise send, not clobber it.
@@ -620,6 +622,7 @@ const CODEX: CliEntry = {
     // `dangerouslyBypassApprovals` on the wire), so it is the one that would have caught a
     // regression; `schema.ts` now rejects a name that is not a declared param.
     privilegedParams: [{ param: 'bypassApprovals', clampTo: false }],
+    mcpConfig: { path: '.codex/config.toml', format: 'codex-toml' },
     // Verified by hand against a real llama.cpp server. Written to an isolated CODEX_HOME
     // so the user's real ~/.codex/config.toml is never touched.
     customModelInjection: {
@@ -721,6 +724,7 @@ const GEMINI: CliEntry = {
     // MATERIALIZE a config (not just touch an already-sent one) or a non-granted owner who
     // sends no geminiConfig at all would still get yolo for free.
     privilegedParams: [{ param: 'approvalMode', clampTo: 'auto_edit', materializeWhenAbsent: true }],
+    mcpConfig: { path: '.gemini/settings.json', format: 'gemini-json' },
     // Web-researched, unverified — needs a restart to pick up (CLI reads these at process
     // start). Confirm the exact model-override env var name against the installed
     // gemini-cli version before shipping.