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_test.go

@@ -486,13 +486,14 @@ func TestMigrateContentHashColumn(t *testing.T) {
 				sender TEXT DEFAULT '',
 				ephemeral INTEGER DEFAULT 0,
 				pinned INTEGER DEFAULT 0,
+				is_template INTEGER DEFAULT 0,
 				replies_to TEXT DEFAULT '',
 				relates_to TEXT DEFAULT '',
 				duplicate_of TEXT DEFAULT '',
 				superseded_by TEXT DEFAULT '',
 				CHECK ((status = 'closed') = (closed_at IS NOT NULL))
 			);
-			INSERT INTO issues SELECT id, title, description, design, acceptance_criteria, notes, status, priority, issue_type, assignee, estimated_minutes, created_at, updated_at, closed_at, external_ref, compaction_level, compacted_at, original_size, compacted_at_commit, source_repo, '', NULL, '', '', '', '', 0, 0, '', '', '', '' FROM issues_backup;
+			INSERT INTO issues SELECT id, title, description, design, acceptance_criteria, notes, status, priority, issue_type, assignee, estimated_minutes, created_at, updated_at, closed_at, external_ref, compaction_level, compacted_at, original_size, compacted_at_commit, source_repo, '', NULL, '', '', '', '', 0, 0, 0, '', '', '', '' FROM issues_backup;
 			DROP TABLE issues_backup;
 		`)
 		if err != nil {