feat: Add project lock mechanism for session exclusivity

New MCP tools:
- project_lock: Lock a project for exclusive session access
- project_unlock: Release a project lock
- project_lock_status: Check lock status for project(s)

Features:
- Automatic lock expiration (default 2h)
- Session ownership verification
- Force unlock option for emergencies
- Lock extension on re-lock by same session

Migration 004: project_locks table with expiry tracking

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

migrations/004_project_locks.sql

@@ -0,0 +1,17 @@
+-- Migration 004: Project locks for session exclusivity
+-- Prevents multiple Claude sessions from working on the same project
+
+CREATE TABLE IF NOT EXISTS project_locks (
+    project TEXT PRIMARY KEY REFERENCES projects(key) ON DELETE CASCADE,
+    session_id TEXT NOT NULL,
+    locked_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    expires_at TIMESTAMP WITH TIME ZONE,
+    reason TEXT
+);
+
+CREATE INDEX IF NOT EXISTS idx_locks_session ON project_locks(session_id);
+CREATE INDEX IF NOT EXISTS idx_locks_expires ON project_locks(expires_at);
+
+-- Record migration
+INSERT INTO schema_migrations (version, applied_at) VALUES ('004_project_locks', NOW())
+ON CONFLICT DO NOTHING;