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/schema.go

@@ -32,6 +32,8 @@ CREATE TABLE IF NOT EXISTS issues (
     ephemeral INTEGER DEFAULT 0,
     -- Pinned field (bd-7h5)
     pinned INTEGER DEFAULT 0,
+    -- Template field (beads-1ra)
+    is_template INTEGER DEFAULT 0,
     -- NOTE: replies_to, relates_to, duplicate_of, superseded_by removed per Decision 004
     -- These relationships are now stored in the dependencies table
     CHECK ((status = 'closed') = (closed_at IS NOT NULL))