feat(daemon): allow idle exit during MCP sessions

[MR-MaoJiu]

Summary

• add daemon.keep_alive_with_mcp, defaulting to true for backward compatibility
• allow users to set it to false so the daemon can reach its normal idle timeout while an MCP client remains connected
• validate the setting as a strict YAML boolean and document it in the README and CCC skill reference

Why

The MCP server currently sends daemon heartbeats for the lifetime of the MCP session. Long-lived editor and agent sessions therefore keep the daemon—and potentially its embedding model memory—resident even when no code search is happening.

With keep_alive_with_mcp: false, only the synthetic MCP heartbeat is disabled. Active requests, indexing, and the existing idle-timeout protections are unchanged. A later real request transparently starts the daemon again through the existing client path.

Compatibility

The default remains true, so existing installations retain their current low-latency behavior unless users explicitly opt into idle exit.

Validation

• uv run prek run --all-files --verbose --show-diff-on-failure
• 301 tests passed; 8 Docker end-to-end tests deselected
• Ruff, Ruff format, gitleaks, uv-lock, and mypy all passed

[badmonster0]

thanks for the contribution! @georgeh0 can help take a look!