fix: Allow tombstones to retain closed_at timestamp

Previously, tombstones could not have a closed_at timestamp due to:
1. Go validation: `if status != closed && closed_at != nil` failed
2. SQL CHECK constraint: `(status = 'closed') = (closed_at IS NOT NULL)`

This caused import failures for tombstones that were closed before being
deleted - a valid scenario where we want to preserve the historical
closed_at timestamp for audit purposes.

Changes:
- internal/types/types.go: Updated validation to allow tombstones with
  closed_at (line 253)
- internal/storage/sqlite/schema.go: Updated CHECK constraint to allow
  closed AND tombstone statuses to have closed_at
- internal/storage/sqlite/migrations/028_tombstone_closed_at.go: Migration
  to update existing databases with the new constraint
- .beads/issues.jsonl: Fixed bd-6s61 status from 'closed' to 'tombstone'

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

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

.beads/issues.jsonl

@@ -104,7 +104,7 @@
 {"id":"bd-6ns7","title":"test hook pin","status":"tombstone","priority":2,"issue_type":"task","assignee":"stevey","created_at":"2025-12-23T04:39:16.619755-08:00","updated_at":"2025-12-23T04:51:29.436788-08:00","deleted_at":"2025-12-23T04:51:29.436788-08:00","deleted_by":"daemon","delete_reason":"delete","original_type":"task"}
 {"id":"bd-6pc","title":"Implement bd pin/unpin commands","description":"Add 'bd pin \u003cid\u003e' and 'bd unpin \u003cid\u003e' commands to toggle the pinned status of issues. Should support multiple IDs like other bd commands.","status":"closed","priority":1,"issue_type":"task","created_at":"2025-12-18T23:33:28.292937-08:00","updated_at":"2025-12-19T17:43:35.713398-08:00","closed_at":"2025-12-19T00:35:31.612589-08:00","dependencies":[{"issue_id":"bd-6pc","depends_on_id":"bd-0vg","type":"blocks","created_at":"2025-12-18T23:33:56.119852-08:00","created_by":"daemon"},{"issue_id":"bd-6pc","depends_on_id":"bd-7h5","type":"blocks","created_at":"2025-12-18T23:34:07.352848-08:00","created_by":"daemon"}]}
 {"id":"bd-6rl","title":"Merge3Way public API does not expose TTL parameter","description":"The public Merge3Way() function in merge.go does not allow callers to configure the tombstone TTL. It hard-codes the default via merge3WayWithTTL(). While merge3WayWithTTL() exists, it is unexported (lowercase). This means the CLI and tests cannot configure TTL at merge time. Use cases: testing with different TTL values, per-repository TTL configuration, debugging with short TTL, supporting --ttl flag in bd merge command (mentioned in design doc bd-zvg). Recommendation: Export Merge3WayWithTTL (rename to uppercase). Files: internal/merge/merge.go:77, 292-298","status":"closed","priority":3,"issue_type":"feature","created_at":"2025-12-05T16:36:15.756814-08:00","updated_at":"2025-12-25T22:45:32.157938-08:00","closed_at":"2025-12-25T22:45:32.157938-08:00","close_reason":"Closed"}
-{"id":"bd-6s61","title":"Version Bump: {{version}}","description":"Release checklist for version {{version}}. This molecule ensures all release steps are completed properly.","status":"closed","priority":1,"issue_type":"epic","created_at":"2025-12-19T22:55:42.487701-08:00","updated_at":"2025-12-25T13:27:50.789625-08:00","closed_at":"2025-12-25T13:27:50.789625-08:00","close_reason":"All children completed","labels":["molecule","template"],"deleted_at":"2025-12-24T16:25:31.251346-08:00","deleted_by":"daemon","delete_reason":"delete","original_type":"epic"}
+{"id":"bd-6s61","title":"Version Bump: {{version}}","description":"Release checklist for version {{version}}. This molecule ensures all release steps are completed properly.","status":"tombstone","priority":1,"issue_type":"epic","created_at":"2025-12-19T22:55:42.487701-08:00","updated_at":"2025-12-25T13:27:50.789625-08:00","closed_at":"2025-12-25T13:27:50.789625-08:00","close_reason":"All children completed","labels":["molecule","template"],"deleted_at":"2025-12-24T16:25:31.251346-08:00","deleted_by":"daemon","delete_reason":"delete","original_type":"epic"}
 {"id":"bd-6sm6","title":"Improve test coverage for internal/export (37.1% → 60%)","description":"The export package has only 37.1% test coverage. Export functionality needs good coverage to ensure data integrity.\n\nCurrent coverage: 37.1%\nTarget coverage: 60%","status":"closed","priority":2,"issue_type":"task","assignee":"beads/alpha","created_at":"2025-12-13T20:43:06.802277-08:00","updated_at":"2025-12-23T22:32:29.16846-08:00","closed_at":"2025-12-23T22:32:29.16846-08:00","close_reason":"Coverage already at 71.8% (target was 60%). Recent commits ba8beb53 and e3e0a044 added tests."}
 {"id":"bd-6ss","title":"Improve test coverage","description":"The test suite reports less than 45% code coverage. Identify the specific uncovered areas of the codebase, including modules, functions, or features. Rank them by potential impact on system reliability and business value, from most to least, and provide actionable recommendations for improving coverage in each area.","status":"closed","priority":2,"issue_type":"task","created_at":"2025-12-18T06:54:23.036822442-07:00","updated_at":"2025-12-18T07:17:49.245940799-07:00","closed_at":"2025-12-18T07:17:49.245940799-07:00"}
 {"id":"bd-70an","title":"test pin","status":"closed","priority":2,"issue_type":"task","created_at":"2025-12-21T11:19:16.760214-08:00","updated_at":"2025-12-21T11:19:46.500688-08:00","closed_at":"2025-12-21T11:19:46.500688-08:00","close_reason":"test issue for pin fix"}