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/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Report incorrect results, crashes, or broken workflows
+title: "[bug] "
+labels: bug
+---
+
+## Summary
+
+<!-- What went wrong? -->
+
+## Expected behavior
+
+## Actual behavior
+
+## Reproduction
+
+<!-- Minimal steps, XML snippet or path under data/cases/, API request JSON, etc. -->
+
+## Environment
+
+- OS:
+- Node (`node -v`):
+- Commit / tag:
+
+## Logs / output
+
+```
+(paste relevant logs)
+```

.gitea/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Propose new physics, API fields, or GUI workflow improvements
+title: "[feature] "
+labels: enhancement
+---
+
+## Problem / motivation
+
+<!-- What engineering workflow is painful today? -->
+
+## Proposal
+
+<!-- Concrete behavior, API shape, or UI change -->
+
+## Acceptance criteria
+
+<!-- What tests / goldens / docs must change? -->
+
+## References
+
+<!-- Papers, field data, or internal notes -->

.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