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.

.opencode/prompts/qa.md

@@ -0,0 +1,28 @@
+# QA Prompt Template
+
+Use this prompt structure for quality assurance:
+
+```
+Read the feature spec at features/PROJ-X-feature-name.md.
+Test the implementation against each acceptance criterion.
+
+Verify:
+- [ ] Each acceptance criterion passes
+- [ ] Edge cases handled gracefully
+- [ ] Error messages are user-friendly
+- [ ] No security vulnerabilities
+- [ ] Performance is acceptable
+
+Document results in the feature spec:
+- [x] AC-1: criterion passed
+- [ ] AC-2: criterion failed (describe bug)
+
+Run: <test command from package.json or composer.json>
+
+Fix any failures before marking as complete.
+```
+
+Example:
+```
+<codex|opencode> exec "Test the login feature: verify email/password validation, test with wrong credentials, check rate limiting. Run npm test. Document results in features/PROJ-1-login.md."
+```