proposal: consolidate optional local dev services into monorepo

Add SPEC.md for eliminating the companion repo (agents-optional-local-dev) by
moving all Docker Compose configs, service configs, and setup automation
directly into the agents monorepo and create-agents-template.

Key points:
- Move configs to optional-services/ in monorepo root
- Rewrite setup-optional.sh to contain full logic (no shim/clone)
- Auto-sync to create-agents-template via lint-staged + rsync
- Update all docs (3 local dev + 5 deployment guides)
- Archive companion repo with redirect README
- User-facing pnpm commands unchanged

See the spec for full surface area inventory, technical design, migration
path, and acceptance criteria.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: nick-inkeep