feat(daemon): shared detached daemon with live file-watch incremental re-index

[sunerpy]

Summary

Introduce a shared, detached background daemon for codegraph serve --mcp with a live file watcher that incrementally re-indexes the project on change — so multiple agent clients share one daemon and the index auto-updates without a manual index run.

Daemon / proxy / watch

• Spawn the shared daemon as a truly detached background process; launcher decision order (CODEGRAPH_NO_DAEMON / internal / no-.codegraph/ / spawn-or-proxy).
• One project watcher per daemon, shared across clients; proxy mode pipes the local handshake to the shared daemon.
• Background catch-up sync on open (non-blocking first call); idle-linger + max-idle backstop; dead-client pid sweep.
• Direct serve mode also runs an in-process watcher.
• Clean degradation on watch-resource exhaustion with bounded backoff.

Config & hooks

• Map custom file extensions to languages via .codegraph/codegraph.json.
• prompt-hook subcommand + opt-in Claude front-load installer hook.

Fixes / polish

• fix(mcp): reopen the cached engine when its DB is replaced on disk.
• perf(daemon): wake the proxy ppid watchdog via condvar instead of poll-sleep.
• refactor(watch): derive the deleted-source check from builtin_language_for_ext; report the real crate version in MCP serverInfo.

Docs

• Document the daemon/watch live-update model, codegraph.json, env vars, and prompt-hook (README + docs/cli.md + docs/mcp.md + architecture.md + zh-CN README).

Verification

• make ci green locally (fmt + clippy -D warnings + full test + guardrail).
• Golden byte-stability preserved (golden_mcp / equivalence / resolution suites pass).
• Hands-on E2E: shared daemon spawn, live incremental re-index (~0.6s, Auto-synced after edit), idle-exit, prompt-hook.