docs: add contributor, security, and forge templates

Add CONTRIBUTING/SECURITY/CHANGELOG, Gitea issue and PR templates, and README guidance for Gitea metadata and releases.

Made-with: Cursor

.gitea/PULL_REQUEST_TEMPLATE.md

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+## Summary
+
+<!-- What does this PR change and why? -->
+
+## Type of change
+
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Docs only
+- [ ] Refactor (no intended behavior change)
+
+## Risk / impact
+
+- [ ] Touches solver numerics (`solver-c`)
+- [ ] Touches API contract (`solver-api`)
+- [ ] Touches GUI (`gui-ts`)
+- [ ] Updates goldens / deterministic hashes
+
+## Testing
+
+```bash
+make test
+```
+
+Paste output or note what could not be run.
+
+## Documentation
+
+- [ ] Updated `Agents/MATH_SPEC.md` (if equations/BCs changed)
+- [ ] Updated `Agents/COMPUTE_PLAN.md` (if roadmap/gaps changed)
+- [ ] Updated `README.md` / `docs/engineering/*` as needed