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/migrations.go

@@ -40,6 +40,7 @@ var migrationsList = []Migration{
 	{"migrate_edge_fields", migrations.MigrateEdgeFields},
 	{"drop_edge_columns", migrations.MigrateDropEdgeColumns},
 	{"pinned_column", migrations.MigratePinnedColumn},
+	{"is_template_column", migrations.MigrateIsTemplateColumn},
 }
 
 // MigrationInfo contains metadata about a migration for inspection
@@ -87,6 +88,7 @@ func getMigrationDescription(name string) string {
 		"migrate_edge_fields":          "Migrates existing issue fields (replies_to, relates_to, duplicate_of, superseded_by) to dependency edges (Decision 004 Phase 3)",
 		"drop_edge_columns":            "Drops deprecated edge columns (replies_to, relates_to, duplicate_of, superseded_by) from issues table (Decision 004 Phase 4)",
 		"pinned_column":                "Adds pinned column for persistent context markers (bd-7h5)",
+		"is_template_column":           "Adds is_template column for template molecules (beads-1ra)",
 	}
 	
 	if desc, ok := descriptions[name]; ok {