feat(types): add template molecules infrastructure for beads-1ra

Add support for template molecules (is_template field and TypeMolecule type):

- Add IsTemplate field to Issue type with JSON support
- Add TypeMolecule constant to IssueType constants
- Add IsTemplate filter to IssueFilter for querying
- Update all SQL queries to include is_template column
- Add migration 024 for is_template column
- Add FindMoleculesJSONLInDir helper for molecules.jsonl path detection

This enables treating certain issues as read-only templates that can be
instantiated to create work items. The template flag allows separating
template molecules from regular work items in queries and exports.

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

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

internal/storage/sqlite/labels.go

@@ -159,7 +159,7 @@ func (s *SQLiteStorage) GetIssuesByLabel(ctx context.Context, label string) ([]*
 		       i.status, i.priority, i.issue_type, i.assignee, i.estimated_minutes,
 		       i.created_at, i.updated_at, i.closed_at, i.external_ref, i.source_repo, i.close_reason,
 		       i.deleted_at, i.deleted_by, i.delete_reason, i.original_type,
-		       i.sender, i.ephemeral, i.pinned
+		       i.sender, i.ephemeral, i.pinned, i.is_template
 		FROM issues i
 		JOIN labels l ON i.id = l.issue_id
 		WHERE l.label = ?