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>

internal/storage/sqlite/schema.go

@@ -36,7 +36,12 @@ CREATE TABLE IF NOT EXISTS issues (
     is_template INTEGER DEFAULT 0,
     -- NOTE: replies_to, relates_to, duplicate_of, superseded_by removed per Decision 004
     -- These relationships are now stored in the dependencies table
-    CHECK ((status = 'closed') = (closed_at IS NOT NULL))
+    -- closed_at constraint: closed issues must have it, tombstones may retain it from before deletion
+    CHECK (
+        (status = 'closed' AND closed_at IS NOT NULL) OR
+        (status = 'tombstone') OR
+        (status NOT IN ('closed', 'tombstone') AND closed_at IS NULL)
+    )
 );
 
 CREATE INDEX IF NOT EXISTS idx_issues_status ON issues(status);