docs: add PR quality contract

.agents/definition_of_done.md

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+# Definition of Done
+
+A completed PR must contain the items below. If an item does not apply, the PR must state `Not applicable` and give the technical reason.
+
+## Required PR Body
+
+The PR body must use `.github/pull_request_template.md` and preserve these sections in this order:
+
+1. `Why does this feature exist?`
+2. `What changed?`
+3. `Behavioural Proof (with video and screenshots)`
+4. `Verification Summary`
+
+## Scope and Implementation
+
+- The reason for the change is stated in user, product, or technical terms.
+- The changed files and behavior are summarized precisely.
+- The implementation avoids unrelated refactors, formatting churn, and hidden scope expansion.
+- New configuration, migrations, permissions, dependencies, or public API changes are called out.
+
+## Behavioural Proof
+
+- UI behavior changes include screenshots from the branch under review.
+- Flows with motion, timing, cursor behavior, audio, or multi-step interaction include video when practical.
+- Screenshots and videos must show the changed behavior, not a generic happy path.
+- Behaviour/E2E tests map to the user-visible behavior changed by the PR.
+- Missing visual proof or E2E coverage is allowed only with a technical reason and a stated replacement verification method.
+
+## Verification
+
+- Format, lint, typecheck, unit, integration, and E2E checks relevant to the changed files are run after the final code change.
+- Test command names and pass/fail results are listed in the PR.
+- Existing unrelated failures are separated from failures introduced by the PR.
+- Any skipped check includes the reason, risk, and owner for follow-up.
+
+## Review Readiness
+
+- Documentation is updated when behavior, setup, or operation changes.
+- Security, privacy, data retention, accessibility, and performance effects are considered when relevant.
+- The PR does not contain secrets, local-only paths, generated junk, or unrelated artifacts.
+- The branch is ready for review only when the PR body contains concrete evidence, not placeholders.

.agents/skills/pr-quality-contract/SKILL.md

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+---
+name: pr-quality-contract
+description: Use before completing work or opening a PR in this repository.
+---
+
+# PR Quality Contract
+
+## Steps
+
+1. Read `.agents/definition_of_done.md` before implementation starts. Completion criterion: the PR scope, required proof, and verification commands are known before coding.
+2. Map behavior changes to tests. Completion criterion: every user-visible behavior changed by the PR has an automated behavior/E2E test or an explicit technical reason it cannot be automated.
+3. Produce behavioral proof for UI changes. Completion criterion: the PR contains current video and screenshots from the changed branch, or states `Not applicable` with the reason.
+4. Run verification on the final branch. Completion criterion: format/lint/type/test/E2E commands relevant to the changed files have been run after the last code change.
+5. Write the PR body using `.github/pull_request_template.md` exactly. Completion criterion: the PR body contains, in order, `Why does this feature exist?`, `What changed?`, `Behavioural Proof (with video and screenshots)`, and `Verification Summary`.
+6. Do not mark work complete with placeholders. Completion criterion: every proof link, screenshot path, video path, test command, failure, skipped check, and residual risk is explicit.
+
+## Evidence Rules
+
+- Test claims require command output.
+- UI behavior claims require screenshots or video from the branch under review.
+- E2E claims require named scenarios and their pass/fail result.
+- Skipped proof requires a technical reason, not convenience.
+- Existing unrelated failures must be separated from failures introduced by the PR.

.github/pull_request_template.md

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+# Why does this feature exist?
+
+-
+
+# What changed?
+
+-
+
+# Behavioural Proof (with video and screenshots)
+
+- Video:
+- Screenshots:
+- Behaviour tests:
+- E2E mapping:
+
+# Verification Summary
+
+- Definition of Done:
+- Commands run:
+- Results:
+- Known risks or skipped checks: