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/skills/help/SKILL.md

@@ -16,15 +16,22 @@ skeleton/
 ├── .gitignore ✓
 ├── AGENTS.md ✓
 ├── .opencode/
-│   ├── settings.json ✓
+│   ├── config/project.md ✓
 │   ├── rules/ ✓
 │   └── skills/ ✓
+├── .codex/
+│   ├── config/project.md ✓
+│   └── prompts/ ✓
+├── .shared/prompts/ ✓
 ├── docs/
-│   ├── doku/ ✓
-│   └── planung/ ✓
-├── www/
-│   ├── api/public/ ✓
-│   ├── game/public/ ✓
-│   └── www/public/ ✓
-└── README.md ✓
-```
+│   └── <domain>/
+│       ├── doku/ ✓
+│       ├── planung/ ✓
+│       └── todos/ ✓
+├── features/
+│   ├── INDEX.md ✓
+│   └── README.md ✓
+└── www/
+    └── <domain>/
+        └── public/ ✓
+```