fix(pin): add 'pinned' field to allowed update fields (gt-zr0a)

The 'bd pin' command was failing with "invalid field for update: pinned"
because the pinned field was missing from allowedUpdateFields.

Fixes:
- Add 'pinned' to allowedUpdateFields in queries.go
- Update importer to include pinned field in updates during import
- Add equalBool comparator for IssueDataChanged to detect pinned changes
- Fix stats query to count pinned=1 instead of status='pinned'
- Fix pinIndicator in list.go to check issue.Pinned instead of status

This unblocks gt mail --pinned functionality.

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

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

internal/importer/importer.go

@@ -554,6 +554,8 @@ func upsertIssues(ctx context.Context, sqliteStore *sqlite.SQLiteStorage, issues
 					updates["acceptance_criteria"] = incoming.AcceptanceCriteria
 					updates["notes"] = incoming.Notes
 					updates["closed_at"] = incoming.ClosedAt
+					// Pinned field (bd-7h5)
+					updates["pinned"] = incoming.Pinned
 
 					if incoming.Assignee != "" {
 						updates["assignee"] = incoming.Assignee
@@ -647,6 +649,8 @@ func upsertIssues(ctx context.Context, sqliteStore *sqlite.SQLiteStorage, issues
 				updates["acceptance_criteria"] = incoming.AcceptanceCriteria
 				updates["notes"] = incoming.Notes
 				updates["closed_at"] = incoming.ClosedAt
+				// Pinned field (bd-7h5)
+				updates["pinned"] = incoming.Pinned
 
 				if incoming.Assignee != "" {
 					updates["assignee"] = incoming.Assignee