fix(import): Update database mtime after import to prevent bd doctor false warnings (#263)

When 'bd sync --import-only' completes, it imports JSONL changes into
the database but doesn't update the database file's modification time.
This causes 'bd doctor' to incorrectly warn that 'JSONL is newer than
database' even when they're in sync.

Root cause: SQLite in WAL mode writes to beads.db-wal; the main beads.db
mtime often doesn't change until a checkpoint. bd doctor compares JSONL
mtime to beads.db mtime, so it can misfire without an mtime bump.

The fix adds touchDatabaseFile() that:
- Only runs when import actually made changes (not dry-run, not unchanged)
- Sets DB mtime to max(JSONL mtime, now) + 1ns to handle clock skew
- Is best-effort (logs warning on failure, doesn't fail import)
- Includes tests for basic touch and clock skew scenarios

Fixes: bd-g3ey

.beads/.gitignore

@@ -18,7 +18,3 @@ bd.db
 !*.jsonl
 !metadata.json
 !config.json
-
-# Exclude merge artifacts (temporary files from 3-way merge)
-beads.base.jsonl
-beads.left.jsonl

.beads/metadata.json

@@ -1,5 +1,5 @@
 {
   "database": "beads.db",
-  "version": "0.21.6",
+  "version": "0.23.0",
   "jsonl_export": "beads.jsonl"
 }