Fix: Preserve timestamps during import (GH-121)

- Change validateBatchIssues() to only set timestamps if IsZero()
- Preserves historical timestamps from external systems (Jira, GitHub)
- Fixes dirty git repo after importing unchanged JSONL
- New issues still get current timestamps as before
- Add daemon.lock to .gitignore

Closes bd-55
Fixes #121

Amp-Thread-ID: https://ampcode.com/threads/T-e53c4a96-38dd-440a-9b8d-824992d33a40
Co-authored-by: Amp <amp@ampcode.com>

.gitignore

@@ -36,6 +36,7 @@ Thumbs.db
 # Daemon runtime files
 .beads/daemon.log
 .beads/daemon.pid
+.beads/daemon.lock
 .beads/bd.sock
 
 # .beads directory files (keep JSONL only)