docs: document V1 quality policy

[tnez]

Parent: #17

Goal

Make the project's quality expectations explicit so future changes preserve the V1 confidence bar.

Suggested Approach

Update the contributor/agent-facing docs with a short policy. Keep it practical and lightweight.

Recommended wording:

Before merging behavior changes, run ./scripts/check. Behavior changes should include a unit test, CLI/example coverage, or a documented reason no automated test fits.

Also mention that ./scripts/check is the local definition of done for code changes.

Scope

• Add or refine the quality policy in AGENTS.md.
• If useful, mirror the testing philosophy in docs/engineering-spec.md under Testing Strategy.
• Keep the policy concise.

Files Likely Touched

• AGENTS.md
• docs/engineering-spec.md only if the spec needs the testing policy too

Non-Goals

• Do not add new tests in this issue.
• Do not change CI in this issue.
• Do not create heavyweight contribution process or PR templates.

Verification

Run:

./scripts/check

This is a docs-only issue, but the repo should remain green.

Acceptance Criteria

• The policy is written in the project docs/agent guidance.
• It is clear what contributors should run locally before calling work done.
• It is clear how behavior changes should be covered.
• The wording stays aligned with the existing engineering principles: small, testable, no over-engineering.