Unify OpenCode and Codex starter kits

- Add .shared/prompts/ with tool-neutral prompts
- Add .opencode/config/project.md, rules/, skills/ from opencode-kit
- Add .codex/config/project.md from codex-kit
- Copy .shared/prompts/ to both .opencode/prompts/ and .codex/prompts/
- Add features/INDEX.md and features/README.md
- Update README with unified structure

The two original starter kits are now redundant.

features/INDEX.md

@@ -0,0 +1,25 @@
+# Feature Index
+
+Tracks all features in the project. Updated by each skill.
+
+| ID | Feature | Status | Spec |
+|----|---------|--------|------|
+| PROJ-1 | User Authentication | Pending | [Spec](PROJ-1-user-auth.md) |
+
+<!-- Add new features above this line -->
+
+---
+
+**Status meanings:**
+- `Pending` — Requirements written, ready for development
+- `Designed` — Architecture completed
+- `Implemented` — Code written, tests passing
+- `QA Passed` — All acceptance criteria met
+- `Deployed` — Live in production
+
+**Workflow:**
+1. `/requirements` → creates spec → status: Pending
+2. `/architecture` → adds tech design → status: Designed
+3. `/php` → implements code → status: Implemented
+4. `/qa` → tests pass → status: QA Passed
+5. `/deploy` → deployed → status: Deployed