Remove deprecated builtin molecules infrastructure

- Delete builtin_molecules.go (empty stubs)
- Remove `mol export` command (exported 0 molecules)
- Clean dead code in catalog.go iterating empty BuiltinMolecules()
- Update docs to reference formula files instead of Go code

Molecules are now defined as .beads/formulas/*.formula.json files
and cooked into proto beads via `bd cook`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/polecat-wisp-architecture.md

@@ -285,8 +285,8 @@ bd mol detach
 
 ### Phase 1: Core Infrastructure (P0)
 
-1. Add `mol-crew-session` to builtin_molecules.go
-2. Add `mol-polecat-session` to builtin_molecules.go
+1. Create `mol-crew-session.formula.json` in `.beads/formulas/`
+2. Create `mol-polecat-session.formula.json` in `.beads/formulas/`
 3. Add wisp attachment mechanism to beads
 4. Update spawn.go for polecat session wisps