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/utils/path.go

@@ -2,6 +2,7 @@
 package utils
 
 import (
+	"os"
 	"path/filepath"
 )
 
@@ -55,6 +56,19 @@ func FindJSONLInDir(dbDir string) string {
 	return filepath.Join(dbDir, "issues.jsonl")
 }
 
+// FindMoleculesJSONLInDir finds the molecules.jsonl file in the given .beads directory.
+// Returns the path to molecules.jsonl if it exists, empty string otherwise.
+// Molecules are template issues used for instantiation (beads-1ra).
+func FindMoleculesJSONLInDir(dbDir string) string {
+	moleculesPath := filepath.Join(dbDir, "molecules.jsonl")
+	// Check if file exists - we don't fall back to any other file
+	// because molecules.jsonl is optional and specific
+	if _, err := os.Stat(moleculesPath); err == nil {
+		return moleculesPath
+	}
+	return ""
+}
+
 // CanonicalizePath converts a path to its canonical form by:
 // 1. Converting to absolute path
 // 2. Resolving symlinks