feat: implement GitLab -> Beads bridge with crash-safe syncing

Adds the core sync engine that maps GitLab events (from lore CLI) to beads
tasks. This is the foundational data layer for Mission Control's unified
task view.

Bridge architecture:
- GitLabBeadMap: JSON file storing event -> bead_id mappings
- MappingKey: Type-safe keys for MR reviews, issues, and authored MRs
- Cursor: Tracks last sync and reconciliation timestamps

Crash-safety features:
- Write-ahead pattern: pending=true written before bead creation
- Atomic file writes via temp file + rename
- Recovery on startup: retries pending entries with bead_id=None
- flock(2) based single-instance locking (prevents concurrent MC)

Two-strike orphan detection:
- First miss sets suspect_orphan=true (items may temporarily vanish)
- Second miss closes the bead (confirmed deleted/merged)
- Reappearance clears the flag (healed)

Sync operations:
- incremental_sync(): Process since_last_check events
- full_reconciliation(): Cross-check all open items
- recover_pending(): Handle interrupted syncs

Dependencies added:
- libc: For flock(2) system call
- thiserror: For ergonomic error types

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -28,6 +28,8 @@ tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 chrono = { version = "0.4", features = ["serde"] }
 dirs = "5"
 notify = "7"
+tauri-plugin-global-shortcut = "2"
+libc = "0.2"
 
 [dev-dependencies]
 tempfile = "3"