perf: unblock startup and Chat loading

[klioen]

Summary

• initialize persisted Worktree watchers in the background with active-first ordering and bounded concurrency
• return newly added Projects after durable catalog persistence instead of waiting for watcher readiness
• open historical Chat sessions by exact ID instead of rescanning all transcripts
• coalesce duplicate in-flight Chat attachments and route buffered events to the latest subscriber
• bound Chat snapshots to the newest 2,000 timeline items before IPC transfer
• add privacy-safe AppState, watcher batch, Project open, and Chat attach timing diagnostics

Safety

• watcher generation/disposal guards prevent late initialization from reviving closed Worktrees
• exact session restore canonicalizes and confines paths to the SDK session root and validates header cwd
• session-bound Pi services and settings are not shared across conversations
• diagnostics contain only fixed codes, durations, counts, booleans, and allowlisted error types

Validation

• focused performance/domain suite: 6 files, 52 tests passed
• pnpm check: 63 files, 390 tests passed; 18 release tests passed
• pnpm bundle: app signature, App smoke, and DMG smoke passed
• git diff --check

[klioen]

Closed at maintainer request; the performance optimization branch is being removed.