add CONTRIBUTING guide

CONTRIBUTING.md

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+# Contributing to Beacon
+
+## Branches
+
+- `main` — stable releases only, protected
+- `dev` — active development, all PRs target this branch
+
+## Workflow
+
+1. Fork or create a branch from `dev`
+2. Make your changes
+3. Open a pull request against `dev`
+4. One commit per PR (squash before opening or use squash merge)
+
+## Commit messages
+
+Use the conventional commits format:
+
+- `feat: add route search endpoint`
+- `fix: correct lat/lon divisor for advert payloads`
+- `chore: update airports.csv`
+- `docs: update README project layout`
+
+## Code style
+
+- Run `gofmt -w .` before committing
+- Run `go vet ./...` — no warnings
+- Run `go build ./...` — must compile
+
+## Tests
+
+- Add tests for any new pure functions
+- Run `go test ./...` before opening a PR
+- Integration tests are not yet required but welcome
+
+## Dependencies
+
+When adding a new dependency please add it to [SHOULDERS.md](SHOULDERS.md) with
+a brief description of what it does.
+
+## Pull requests
+
+- Target `dev` not `main`
+- One logical change per PR
+- Include a brief description of what changed and why
+- Reference any related issues
+
+## Releases
+
+Merges from `dev` to `main` are done by maintainers and represent a versioned
+release.