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/architecture.md

@@ -0,0 +1,21 @@
+# Architecture Prompt Template
+
+Use this prompt structure for technical design:
+
+```
+Read the feature spec at features/PROJ-X-feature-name.md.
+Design the technical architecture and add a Tech Design section to the spec file.
+
+Focus on:
+- Component structure (for frontend)
+- API design (endpoints, request/response shapes)
+- Database schema (tables, relationships, indexes)
+- Implementation order (what to build first, dependencies)
+- Security considerations
+- Tech stack decisions (libraries, patterns)
+```
+
+Example:
+```
+<codex|opencode> exec "Read features/PROJ-1-login.md. Design the auth architecture: JWT vs sessions, database schema for users, password hashing approach, and implementation order."
+```