[Juliusolsson05]

Summary

Closes #155.

Adds Agent Code's built-in MCP host: a loopback Streamable HTTP MCP server owned by the main process, plus session launch wiring so Claude/Codex agents can be started with scoped Agent Code MCP access.

This PR only adds the foundation and the ping domain. It does not add orchestration tools; those live in the stacked orchestration PR.

Behavior

• Main starts one local MCP HTTP host on 127.0.0.1.
• Each session that asks for built-in MCP domains gets a per-session bearer token and MCP server config.
• Providers receive concrete launch config only:
  • Claude gets --mcp-config JSON.
  • Codex gets --config mcp_servers.* flags.
• Closing/stopping a session revokes its token.
• The first built-in domain is ping, exposing agent_code_ping so a launched agent can confirm its scoped session id, cwd, and domains.

Design

The host lives in Electron main instead of a separate stdio helper process. That avoids packaged-app problems where Agent Code would need to locate a separate Node binary just to run its own MCP server, and it gives future built-in domains direct access to app services through typed dependencies rather than rediscovery.

The session metadata remains the policy source (builtInMcpDomains). Providers only receive the minted URL/header config for the current process, so a provider runtime does not need to know about long-lived domain policy.

Files

• src/mcp/shared/types.ts — built-in domain/config types and domain normalization.
• src/mcp/runtime/BuiltInMcpHttpHost.ts — loopback HTTP host, token registry, request authentication, per-request McpServer lifecycle.
• src/mcp/runtime/createBuiltInMcpServer.ts — builds the scoped MCP server and registers agent_code_ping.
• src/main/sessionManager.ts — registers/revokes built-in MCP configs during session spawn/stop.
• src/providers/claude/runtime/claudeSession.ts and src/providers/codex/runtime/codexSession.ts — inject provider-specific MCP launch flags.
• src/renderer/src/workspace/types.ts, src/shared/types/session.ts, preload/session API wiring — carries builtInMcpDomains / builtInMcpServers through the existing spawn path.

Verification

• agent_code_ping was live-exercised from an MCP-enabled agent during development and returned the expected scoped session/cwd/domain payload.
• tsc -b was attempted on the branch. The MCP files typecheck, but the repo still reports unrelated pre-existing errors from submodule type resolution and BlockRow.tsx/mergedEntries.ts issues already present outside this PR.
• No test files added, per repo policy.

Test Plan

• Spawn a Claude session with builtInMcpDomains: ['ping']; confirm agent_code_ping is available and returns the current session id/cwd.
• Spawn a Codex session with builtInMcpDomains: ['ping']; confirm the same.
• Close the session and confirm the old MCP URL/token returns 401.
• Spawn a normal session with no built-in domains; confirm no Agent Code MCP config is injected.

Follow-Up

The stacked orchestration PR builds on this foundation by adding an orchestration domain and the tools that create/list/prompt child agents. The AI Workspace work in #157 should also build on this host, but this PR leaves #157 for a separate implementation.