Add watcher preopen controls and harden server shutdown

[isham703]

Summary

This PR addresses Swift MCP memory/process buildup observed in large workspaces by adding opt-in runtime controls and hardening shutdown behavior, while preserving existing defaults.

Changes

• Add runtime flags/env controls (default-preserving):
  • --watcher-preopen-on-register (default true)
  • --watcher-preopen-max-files (default 0 = unlimited)
  • --idle-timeout (default 0s = disabled)
• Add watcher config fields:
  • PreopenOnRegistration bool
  • PreopenMaxFiles int
• Gate registration-time preopen scans behind PreopenOnRegistration.
• Enforce deterministic preopen cap via PreopenMaxFiles (counts successful opens only).
• Add idle watchdog and activity tracking using MCP BeforeAny hook.
• Make server cleanup idempotent (sync.Once) and ensure cleanup always runs when stdio loop exits.
• Make Client.Close() idempotent in LSP client to avoid repeated wait/close races.
• Update README with new flags/env and Codex-specific example.

Why

In practice, duplicated MCP stacks plus registration-triggered preopen scans can create high memory pressure and process accumulation. This change gives clients an opt-out/cap for preopen and adds reliable idle/process cleanup.

Compatibility

Defaults preserve current behavior globally, so existing clients are unaffected unless they opt in.

Tests

• Added/updated unit coverage for:
  • preopen disabled => no registration scan opens
  • preopen enabled => matching files open
  • preopen max cap => bounded opens
  • repeated close calls => idempotent/no-op-safe
• Ran package tests (non-integration) successfully.

Files

• main.go
• internal/watcher/interfaces.go
• internal/watcher/watcher.go
• internal/lsp/client.go
• internal/lsp/client_close_test.go
• internal/watcher/testing/mock_client.go
• internal/watcher/testing/watcher_test.go
• README.md