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

CONTRIBUTING.md

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+# Contributing
+
+Thanks for helping improve this project. The goal is **deterministic, explainable** rod-string / dynamometer simulation: C is the numerical source of truth; Node orchestrates; the GUI is workflow and visualization.
+
+## Before you start
+
+Read (in order):
+
+1. [AGENTS.md](AGENTS.md)
+2. [Agents/MATH_SPEC.md](Agents/MATH_SPEC.md)
+3. [Agents/COMPUTE_PLAN.md](Agents/COMPUTE_PLAN.md)
+
+## Local development
+
+### Prerequisites
+
+- `gcc`, `make`
+- Node.js 20+ and `npm`
+- Docker (optional, for compose / CI image)
+
+### Build C drivers (JSON on stdin)
+
+From the repository root, follow the `gcc` commands in [README.md](README.md) or use CMake:
+
+```bash
+cmake -S solver-c -B solver-c/build
+cmake --build solver-c/build
+```
+
+The API expects `solver-c/solver_main` and `solver-c/solver_fea_main` in the repo root layout (see `docker-compose.yml` and `solver-api`).
+
+### Run tests
+
+```bash
+make test
+```
+
+This runs `solver-api` Vitest, `gui-ts` Vitest, and `solver-c/test_solver`.
+
+## Pull requests
+
+- Keep changes focused; avoid drive-by refactors.
+- If you change solver equations or boundary conditions, update [Agents/MATH_SPEC.md](Agents/MATH_SPEC.md) and the gap table in [Agents/COMPUTE_PLAN.md](Agents/COMPUTE_PLAN.md).
+- If you change the JSON contract or HTTP responses, bump `schemaVersion` only with an explicit migration plan (see [AGENTS.md](AGENTS.md)).
+
+## Coding style
+
+- Match existing formatting and naming in each package (`solver-c`, `solver-api`, `gui-ts`).
+- Prefer small, test-backed changes for numerics.
+
+## Security
+
+See [SECURITY.md](SECURITY.md).