refactor(db): move ticketing database to shared worktree location

The SQLite database now lives at ../settledreach.db, shared across all
git worktrees. This eliminates binary merge conflicts that occurred
whenever two branches modified the database independently.

All Python connectors (ticket CLI, sqlite_connector, decisions_sync)
now resolve the DB path relative to their own location — three levels
up from db/connectors/ reaches the worktree parent. The _main_worktree()
git-rev-parse pattern and config.json sqlite_db key are removed.

The database file is untracked from git; a backup mechanism commits
snapshots through the main branch (see following commits).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -8,9 +8,8 @@ client/export/
 client/reports/
 client/*.import
 
-# Database journal files (transient, not the DB itself)
-db/commonwealth.db-wal
-db/commonwealth.db-shm
+# Database (shared across worktrees at ../settledreach.db, not tracked)
+db/commonwealth.db*
 
 # Python
 __pycache__/