docs: add Contributing section to README

[Param-Harrison]

Summary

Adds a "Contributing" section to the README explaining how to propose changes and open pull requests for this repository.

What changed

• README.md — new ## Contributing section with a numbered guide covering fork → setup → check permissions → make focused changes → run checks → open PR

Key constraints followed

• Uses only commands the repository already documents (bun install, per-area test commands from the Checks section)
• No new commands, scripts, or infrastructure invented
• Change is limited to README.md (documentation only)
• References .factory/README.md and AGENTS.md for the full rules

Verification

• Change touches only README.md
• All references point to existing commands documented elsewhere in the README
• No test changes needed (documentation-only change)

Closes #638

[Param-Harrison]

Verdict: approve

Findings

Correctness. The PR adds a ## Contributing section to README.md between the existing ## Checks and ## License sections (24 line additions, 0 deletions). Every command referenced (bun install, the four per-area test commands) is documented in the existing README. Every file referenced (.factory/README.md, .factory/targets.json, AGENTS.md, LICENSE) exists on disk at HEAD. The content is accurate and consistent with the repository's documented workflows.

Tests. Documentation-only change. No automated tests validate README content in this repository. A documentation change that introduces no behavior, executable code, or new dependencies requires no test suite additions — the handoff records this as the affirmative reason no tests are needed.

Scope. Single file, single topic. The diff is focused and coherent. Closes #638 as claimed.

Pattern consistency. The section follows the existing README tone (direct, minimal, no padding) and formatting conventions (ATX headers, numbered lists for steps, inline code for commands). References to .factory/README.md and AGENTS.md match the pattern set in the existing opening paragraph. The insertion point (between Checks and License) is natural. No deviations from established patterns.

Issue and intent

• Issue: Add a Contributing section to the README #638 "Add a Contributing section to the README" (OPEN, labels: status: auto-triaged, effort:low, impact:low)
• PR classification: docs/maintenance — documentation-only change
• Behavior/scope match: The PR implements exactly what the issue asks: a Contributing section guiding contributors through fork → setup → check permissions → make focused changes → run checks → open PR, using only existing commands and keeping changes to README.md

Verification

Commands executed:

bun install                                  — success (372ms)

No further test execution is warranted for a single-file documentation change. The PR changes nothing that executes — no package.json scripts, no lockfiles, no test config, no CI workflows. Security inspection cleared: no executable infrastructure altered.

The PR is MERGEABLE with mergeStateStatus: CLEAN. No merge conflicts.

Existing review disposition

None. No prior reviews, comments, or unresolved threads on this PR.

Adversarial check

Strongest request-changes argument: a contributor following the guide encounters a broken or incorrect command. The argument fails because every command and reference in the added section was verified against the actual codebase at HEAD — all commands exist in the README, all linked files exist, and the content is factually correct.

Assumptions

• The insertion point (between Checks and License) is the natural location for a Contributing section, consistent with standard README layout conventions. No issue discussion specified positioning.
• Documentation-only changes in a repository without README-testing infrastructure do not require automated verification beyond confirming the content's factual accuracy.
• The absence of CI checks on this branch is expected for a PR opened without triggering CI (no CI config exists on this branch). Not a defect.

Open questions

None.

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Review runtime: openrouter/deepseek/deepseek-v4-flash, reasoning setting: off.