feat(import,sync): add --no-git-history flag to prevent spurious deletions

Fixes bd-0b2: The git history backfill mechanism was causing data loss
during JSONL filename migrations (beads.jsonl → issues.jsonl). When issues
existed in the old filename's git history, the backfill incorrectly treated
them as "deleted" and purged them from the database.

Changes:
- Add NoGitHistory field to importer.Options and ImportOptions structs
- Modify purgeDeletedIssues() to skip git history check when flag is set
- Add --no-git-history flag to bd import command
- Add --no-git-history flag to bd sync command
- Update purge_test.go to pass Options argument

Usage:
  bd import -i .beads/issues.jsonl --no-git-history
  bd sync --no-git-history

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.beads/issues.jsonl

@@ -1,6 +1,6 @@
 {"id":"bd-03r","title":"Document deletions manifest in AGENTS.md and README","description":"Parent: bd-imj\n\n## Task\nAdd documentation about the deletions manifest feature.\n\n## Locations to Update\n\n### AGENTS.md\n- Explain that deletions.jsonl is tracked in git\n- Document that `bd delete` records to the manifest\n- Explain cross-clone propagation mechanism\n\n### README.md  \n- Brief mention in .beads directory structure section\n- Link to detailed docs if needed\n\n### docs/deletions.md (new file)\n- Full technical documentation\n- Format specification\n- Pruning policy\n- Git history fallback\n- Troubleshooting\n\n## Acceptance Criteria\n- AGENTS.md updated with deletion workflow\n- README.md mentions deletions.jsonl purpose\n- New docs/deletions.md with complete reference","status":"closed","priority":2,"issue_type":"task","created_at":"2025-11-25T14:56:49.13027-08:00","updated_at":"2025-11-25T15:17:23.145944-08:00","closed_at":"2025-11-25T15:17:23.145944-08:00"}
 {"id":"bd-055","title":"Fix hyphenated prefix parsing in ExtractIssuePrefix (GH #395)","description":"","status":"closed","priority":2,"issue_type":"bug","created_at":"2025-11-26T15:22:22.395177-08:00","updated_at":"2025-11-26T16:46:57.8927-08:00","closed_at":"2025-11-26T16:46:57.8927-08:00"}
-{"id":"bd-0b2","title":"Need --no-git-history flag to disable git history backfill during import","description":"During JSONL migration (beads.jsonl → issues.jsonl), the git history backfill mechanism causes data loss by finding issues in the old beads.jsonl git history and incorrectly treating them as deleted.\n\nA --no-git-history flag for 'bd import' and 'bd sync' would allow users to disable the git history fallback when it's causing problems.\n\nUse cases:\n- JSONL filename migrations\n- Repos with complex git history\n- Debugging import issues\n- Performance (skip slow git scans)\n\nRelated: bd-0gh (migration causes spurious deletions)","status":"open","priority":2,"issue_type":"feature","created_at":"2025-11-26T22:28:22.5286-08:00","updated_at":"2025-11-26T22:28:22.5286-08:00"}
+{"id":"bd-0b2","title":"Need --no-git-history flag to disable git history backfill during import","description":"During JSONL migration (beads.jsonl → issues.jsonl), the git history backfill mechanism causes data loss by finding issues in the old beads.jsonl git history and incorrectly treating them as deleted.\n\nA --no-git-history flag for 'bd import' and 'bd sync' would allow users to disable the git history fallback when it's causing problems.\n\nUse cases:\n- JSONL filename migrations\n- Repos with complex git history\n- Debugging import issues\n- Performance (skip slow git scans)\n\nRelated: bd-0gh (migration causes spurious deletions)","status":"closed","priority":2,"issue_type":"feature","created_at":"2025-11-26T22:28:22.5286-08:00","updated_at":"2025-11-26T23:09:29.398357-08:00","closed_at":"2025-11-26T23:09:29.398357-08:00"}
 {"id":"bd-1pj6","title":"Proposal: Custom status states via config","description":"Proposal to add 'custom status states' via `bd config`.\nUsers could define an optional issue status enum (e.g., awaiting_review, review_in_progress) in the config.\nThis would enable multi-step pipelines to process issues where each step correlates to a specific status.\n\nExamples:\n- awaiting_verification\n- awaiting_docs\n- awaiting_testing\n","status":"open","priority":3,"issue_type":"feature","created_at":"2025-11-20T18:55:48.670499-05:00","updated_at":"2025-11-20T18:55:48.670499-05:00"}
 {"id":"bd-2em","title":"Expand checkHooksQuick to verify all hook versions","description":"Currently checkHooksQuick only checks post-merge hook version. Should also check pre-commit, pre-push, and post-checkout for completeness. Keep it lightweight but catch more outdated hooks.","status":"closed","priority":2,"issue_type":"task","created_at":"2025-11-25T19:27:47.432243-08:00","updated_at":"2025-11-25T19:50:21.378464-08:00","closed_at":"2025-11-25T19:50:21.378464-08:00"}
 {"id":"bd-39o","title":"Rename last_import_hash metadata key to jsonl_content_hash","description":"The metadata key 'last_import_hash' is misleading because it's updated on both import AND export (sync.go:614, import.go:320).\n\nBetter names:\n- jsonl_content_hash (more accurate)\n- last_sync_hash (clearer intent)\n\nThis is a breaking change requiring migration of existing metadata values.","status":"open","priority":2,"issue_type":"task","created_at":"2025-11-20T21:31:07.568739-05:00","updated_at":"2025-11-20T21:31:07.568739-05:00"}