[Feature Request] Add a CONTRIBUTING.md consolidating the de-facto contribution rules #603

Problem

There is no CONTRIBUTING.md in the repository. The only documented contribution guidance is the five-line "Contributing" section in the README (fork → branch → commit → push → PR). The actual expectations a PR has to satisfy are spread across AGENTS.md, docs/DEVELOPMENT.md, and CLAUDE.md (symlink), and a first-time contributor has no way to know them without reading the whole development docs.

The project clearly has de-facto contribution gates — most are visible from the release notes and code review conventions — but they are not written down as contribution rules anywhere.

Proposal

Add a short CONTRIBUTING.md (or a Contributing section in docs/) that consolidates the rules that already govern the codebase:

Workflow

• Fork → feature branch → PR (already in README).
• Run checks before opening a PR: bun test and bun run lint must be green.
• Use bun run lint / bun run lint:fix only — never invoke eslint/tsc directly; never disable a lint rule via a comment.

Compatibility guarantees a PR must not break

• dist/ must stay compatible with Node.js 14+.
• Dual runtime (Bun + Node.js) must keep working.
• React and React DOM are exact-version pins; dependency updates change package.json and bun.lock together.
• The ink@6.2.0 patch (patches/) must be preserved when touching dependencies.
• Keep the macOS backspace patch rationale in mind when upgrading ink.

Testing expectations

• Vitest via Bun (bun test); new widgets/features come with tests under src/widgets/__tests__/ / src/utils/__tests__/.
• Usage-fetch tests must be sandboxed (HOME, USERPROFILE, CLAUDE_CONFIG_DIR, proxy vars set explicitly) so they never touch a developer's live ~/.cache/ccstatusline.

Behavioral invariants

• Settings saves stay atomic and write through symlinked settings.json.
• Invalid/unreadable settings files are never overwritten.
• Manual verification guidance: piped-JSON mode for renderer changes, TUI for UI changes.

This would also help agent-based contributors (Claude Code and similar) — the content overlaps with AGENTS.md, but a CONTRIBUTING.md is the canonical entry point humans look for first, and AGENTS.md could simply link to it.