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/storage/sqlite/queries.go

@@ -558,6 +558,8 @@ var allowedUpdateFields = map[string]bool{
 	// Messaging fields (bd-kwro)
 	"sender":    true,
 	"ephemeral": true,
+	// Pinned field (bd-7h5)
+	"pinned": true,
 	// NOTE: replies_to, relates_to, duplicate_of, superseded_by removed per Decision 004
 	// Use AddDependency() to create graph edges instead
 }