Fix auto-import collision detection and enforce status/closed_at invariant (bd-226)

Code review and fixes:
- Increased scanner buffer to 2MB for large JSON lines
- Added line numbers and snippets to parse error messages
- Made non-SQLite fallback conservative (skip import to prevent data loss)
- Improved collision warnings (concise, show first 10 IDs)
- Removed unused autoImportWithoutCollisionDetection function

Status/closed_at invariant enforcement:
- Auto-import now enforces invariant on all creates/updates
- Fixed CreateIssue to respect closed_at field (was ignoring it)
- Closed issues without closed_at get timestamp set automatically

Integration tests:
- TestAutoImportWithCollision: verifies local changes preserved
- TestAutoImportNoCollision: happy path with new issues
- TestAutoImportClosedAtInvariant: enforces invariant

Closes bd-226, bd-230, bd-231

internal/storage/sqlite/sqlite.go

@@ -406,14 +406,14 @@ func (s *SQLiteStorage) CreateIssue(ctx context.Context, issue *types.Issue, act
 		INSERT INTO issues (
 			id, title, description, design, acceptance_criteria, notes,
 			status, priority, issue_type, assignee, estimated_minutes,
-			created_at, updated_at, external_ref
-		) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
+			created_at, updated_at, closed_at, external_ref
+		) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
 	`,
 		issue.ID, issue.Title, issue.Description, issue.Design,
 		issue.AcceptanceCriteria, issue.Notes, issue.Status,
 		issue.Priority, issue.IssueType, issue.Assignee,
 		issue.EstimatedMinutes, issue.CreatedAt, issue.UpdatedAt,
-		issue.ExternalRef,
+		issue.ClosedAt, issue.ExternalRef,
 	)
 	if err != nil {
 		return fmt.Errorf("failed to insert issue: %w", err)