Add Stack Overflow Omi integration app

plugins/omi-stack-overflow-app/Procfile

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+web: uvicorn main:app --host 0.0.0.0 --port ${PORT:-8080}

plugins/omi-stack-overflow-app/README.md

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+# Stack Overflow x Omi
+
+Search Stack Overflow and read top answers from Omi conversations.
+
+This is a standalone no-auth Omi app backed by the public Stack Exchange API. It does not require environment variables, OAuth, or a Stack Exchange API key.
+
+## Tools
+
+- `search_questions`: search Stack Overflow or another Stack Exchange site by query, optional tags, and accepted-answer status.
+- `get_question`: fetch a specific question by ID with title, metadata, tags, link, and body excerpt.
+- `get_top_answers`: fetch the highest-voted answers for a question.
+
+## Local Development
+
+```bash
+python -m venv .venv
+source .venv/bin/activate
+pip install -r requirements.txt
+uvicorn main:app --reload --port 8080
+```
+
+Health check:
+
+```bash
+curl http://localhost:8080/health
+```
+
+Tool manifest:
+
+```bash
+curl http://localhost:8080/.well-known/omi-tools.json
+```
+
+Search Stack Overflow:
+
+```bash
+curl -X POST http://localhost:8080/tools/search_questions \
+  -H "Content-Type: application/json" \
+  -d '{"query":"fastapi dependency injection", "tags":"python;fastapi", "limit":3}'
+```
+
+Fetch top answers:
+
+```bash
+curl -X POST http://localhost:8080/tools/get_top_answers \
+  -H "Content-Type: application/json" \
+  -d '{"question_id":11227809, "limit":2}'
+```
+
+## Deployment
+
+The app is ready for Railway or Heroku-style deployment:
+
+- `Procfile` starts uvicorn on `$PORT`
+- `railway.toml` defines the same start command and `/health` check
+- `runtime.txt` pins Python 3.11
+
+## Notes
+
+Unauthenticated Stack Exchange API calls are rate limited by Stack Exchange. The app keeps responses concise and caps result limits to reduce quota usage.