docs: add reviewer evidence checklist (#145)

[teihin]

Description

Closes #145.

Adds a maintainer-facing reviewer evidence checklist for Aegis Contracts PRs and links it from the README Contributor Guides section. The checklist focuses on evidence reviewers should verify before approving, merging, or marking a contribution evaluation-ready.

Completion Table

Acceptance Criterion	Status	Implementation Evidence	Test Evidence
Reviewer evidence checklist is added.	Complete	Added docs/reviewer-evidence-checklist.md.	Documentation-only; verified via diff review.
Implementation scope is checked.	Complete	Added Scope Evidence section covering issue link, unrelated diffs, generated files, and touched-file rationale.	Documentation-only; verified in new checklist.
Test evidence is checked.	Complete	Added Test Evidence section covering happy paths, negative paths, fixtures, and no-test justification.	Documentation-only; verified in new checklist.
CI status is checked.	Complete	Added CI and Local Verification Evidence section covering commands, GitHub Actions, failures, and reviewer reproduction.	Documentation-only; verified in new checklist.
Acceptance criteria are checked.	Complete	Added Acceptance-Criteria Evidence section requiring reviewer comparison against issue criteria.	Documentation-only; verified in new checklist.
README links to reviewer guidance.	Complete	Added README link to docs/reviewer-evidence-checklist.md in Contributor Guides.	`rg -n "Reviewer Evidence Checklist

Type of Change

• Documentation update

PR Evidence Checklist

1. Issue Reference

• This PR links to the issue being addressed: Closes Add Aegis Contracts PR reviewer evidence checklist #145.

2. Implementation Summary

• Added a concise reviewer evidence checklist.
• Updated README with the new reviewer guidance link.
• No public functions, events, error codes, roles, storage keys, or runtime behavior were changed.

3. Tests Added or Justification

• No executable tests were added because this is a documentation-only change.
• The changed files are limited to README.md and docs/reviewer-evidence-checklist.md.

4. Commands Run

• git diff --cached --check
• git diff HEAD~1..HEAD --check
• rg -n "Reviewer Evidence Checklist|reviewer-evidence-checklist" README.md docs/reviewer-evidence-checklist.md

5. CI Status

• GitHub Actions pending after PR creation.

6. Acceptance Criteria Coverage

• Every acceptance criterion is mapped in the Completion Table above.

Policy & Standards

• Documentation avoids implying payout approval; the checklist separates merge readiness from reward evaluation.
• No Rust/Soroban code was changed, so cargo fmt, cargo clippy, and cargo test are not applicable to this PR.