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/beads/beads.go

@@ -211,11 +211,12 @@ const (
 
 // IssueType constants
 const (
-	TypeBug     = types.TypeBug
-	TypeFeature = types.TypeFeature
-	TypeTask    = types.TypeTask
-	TypeEpic    = types.TypeEpic
-	TypeChore   = types.TypeChore
+	TypeBug      = types.TypeBug
+	TypeFeature  = types.TypeFeature
+	TypeTask     = types.TypeTask
+	TypeEpic     = types.TypeEpic
+	TypeChore    = types.TypeChore
+	TypeMolecule = types.TypeMolecule
 )
 
 // DependencyType constants