docs: add contributing guide and code of conduct (#562)

Addresses community request for contributor guidelines.
Adds CONTRIBUTING.md with setup instructions, PR workflow, model
addition guide, and code guidelines. Adds CODE_OF_CONDUCT.md
referencing the Contributor Covenant v2.1.

Closes discussion #460

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CODE_OF_CONDUCT.md

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+# Code of Conduct
+
+This project adopts the **Contributor Covenant v2.1** as its code of conduct.
+
+The full text is available at: https://www.contributor-covenant.org/version/2/1/code_of_conduct/
+
+## Summary
+
+We are committed to making participation in this project a welcoming experience for everyone regardless of background or identity. We expect all contributors to act professionally and respectfully.
+
+## Reporting
+
+If you experience or witness behavior that violates this code of conduct, please report it by opening an issue or contacting the maintainers directly.
+
+## Enforcement
+
+Project maintainers are responsible for clarifying standards of acceptable behavior and will take appropriate and fair corrective action in response to any behavior they deem inappropriate.

CONTRIBUTING.md

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+# Contributing to llmfit
+
+Thanks for your interest in contributing! Whether it's a bug fix, new feature, model addition, or documentation improvement, we appreciate the help.
+
+## Getting started
+
+### Prerequisites
+
+- **Rust** (stable toolchain, edition 2024, MSRV 1.85+)
+- **Python 3** (for model database scripts — stdlib only, no pip dependencies)
+- **Git**
+
+### Building from source
+
+```sh
+git clone https://github.com/AlexsJones/llmfit.git
+cd llmfit
+make build        # debug build
+make release      # release build
+```
+
+### Running
+
+```sh
+make run                      # TUI mode (default)
+cargo run -- --cli            # classic table output
+cargo run -- system           # show detected hardware
+cargo run -- fit --perfect    # show best-fit models
+cargo run -- search "llama"   # search models
+```
+
+### Useful commands
+
+```sh
+make test       # run all tests
+make fmt        # format code (cargo fmt)
+make clippy     # run linter (cargo clippy)
+make check      # fast compilation check
+```
+
+## Project structure
+
+llmfit is a Rust workspace with three crates:
+
+| Crate | Description |
+|-------|-------------|
+| `llmfit-core` | Core library — hardware detection, model database, fit analysis |
+| `llmfit-tui` | Terminal user interface (ratatui + crossterm) |
+| `llmfit-desktop` | Desktop app (Tauri) |
+
+Supporting directories:
+
+- `scripts/` — Python utilities for scraping HuggingFace and Docker model metadata
+- `data/` — Generated JSON model databases (do not edit manually)
+- `llmfit-python/` — Python bindings
+- `llmfit-web/` — Web interface
+
+For a deeper dive into the architecture, see [AGENTS.md](AGENTS.md).
+
+## How to contribute
+
+### Reporting bugs
+
+Open an [issue](https://github.com/AlexsJones/llmfit/issues) with:
+
+- What you expected to happen
+- What actually happened
+- Your OS, hardware (GPU model, RAM), and llmfit version (`llmfit --version`)
+- Steps to reproduce
+
+### Suggesting features
+
+Start a [discussion](https://github.com/AlexsJones/llmfit/discussions) or open an issue. We're happy to chat about ideas before you invest time coding.
+
+### Submitting a pull request
+
+1. **Fork** the repo and create a branch from `main`.
+2. Make your changes.
+3. Run `cargo fmt` — most CI failures are from unformatted code.
+4. Run `make clippy` and fix any warnings.
+5. Run `make test` to verify nothing is broken.
+6. Open a PR against `main` with a clear description of what and why.
+
+Keep PRs focused. One bug fix or feature per PR is easier to review than a combined change.
+
+### Adding a new model
+
+1. Add the model's HuggingFace repo ID (e.g., `meta-llama/Llama-3.1-8B`) to the `TARGET_MODELS` list in `scripts/scrape_hf_models.py`.
+2. If the model is gated (requires HF authentication), add a fallback entry to the `FALLBACKS` dict in the same script.
+3. Run `make update-models` to regenerate the database and rebuild.
+4. Verify with `./target/release/llmfit list`.
+5. Update [MODELS.md](MODELS.md) if needed.
+6. Open a PR.
+
+## Code guidelines
+
+- **No `unsafe` code.**
+- No `.unwrap()` on user-facing paths. Use proper error handling or `expect()` with a descriptive message for internal invariants only.
+- Keep TUI rendering stateless — `tui_ui::draw()` must not mutate application state.
+- Prefer well-maintained crates with minimal transitive dependencies.
+- The Python scraper uses only stdlib (`urllib`, `json`). Do not add pip dependencies.
+
+## Code of conduct
+
+This project follows the [Contributor Covenant Code of Conduct](CODE_OF_CONDUCT.md). By participating, you are expected to uphold this code. Please report unacceptable behavior by opening an issue.
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the [MIT License](LICENSE).