docs: add reviewer evidence checklist

[CleanDev-Fix]

Closes #542

Summary

Adds a maintainer-side reviewer evidence checklist for deciding whether a
PocketPay Contracts PR is complete and evaluation-ready. The guide organizes
scope, implementation, tests, exact-head CI, acceptance criteria,
documentation/risk, and evidence-specific review outcomes.

Changes

• add docs/reviewer-evidence-checklist.md, linking the existing quality,
  traceability, testing, security, and documentation-style sources;
• add the reviewer guide to the README project tree and documentation index;
  and
• link the guide from the maintainer workflow in CONTRIBUTING.md.

Contract Functions Changed

None. This is a documentation-only change.

Tests Added or Updated

None. No executable or contract behavior changed.

Security Considerations

No security impact. The checklist points reviewers to the existing contract
security source and does not alter balances, authorization, storage, events,
external calls, dependencies, or workflows.

Commands Run

• git diff --check — PASS (line-ending warnings only; no whitespace errors)
• six acceptance-criteria content assertions — PASS
• repository-relative Markdown link validation — PASS
• independent QA — PASS, no blocking or actionable findings

The Rust/contract suite was not run because the patch changes Markdown only.

CI Status

This PR is intentionally a draft until relevant hosted checks are visible and
green.

Traceability Table

Criterion ID	Criterion Text	Changed Function(s)	Test(s)	Edge Cases Covered	Status
AC-1	Reviewer evidence checklist is added	None; docs/reviewer-evidence-checklist.md added	N/A — documentation-only	Copy-ready outcome record included	Met
AC-2	Implementation scope is checked	None; issue/scope and implementation/risk sections added	Content assertion	Unrelated work, placeholders, and ambiguous scope covered	Met
AC-3	Test evidence is checked	None; named tests, paths, failure/boundary coverage, and no-test justification covered	Content assertion	Skipped/flaky/failing evidence cannot be called passing	Met
AC-4	CI status is checked	None; exact-head jobs, nonpassing states, infra ownership, and draft handling covered	Content assertion	Automation dispatch and zero-job workflows are not treated as patch-green	Met
AC-5	Acceptance criteria are checked	None; traceability rows, documentation impact, and Partial/Not Met handling covered	Content assertion	Checked boxes require matching evidence	Met
AC-6	README links to reviewer guidance	None; project tree and Documentation index updated	Relative-link check	Link resolves from README	Met

Contributor Self-Review

• Every issue acceptance criterion is mapped above.
• The new guide is maintainer-side and links existing sources instead of
  duplicating their details.
• No contract, CI workflow, reward, payment, dependency, or security
  behavior changes.
• Focused diff/content/link checks and independent QA pass.
• Hosted CI is pending; this PR remains a draft.

Known limitation: the Rust/contract suite was not run because no executable
behavior changed.

[El-swaggerito]

\nThis PR is currently a draft.\n\nThe auto-merge automation will not merge draft pull requests.

[CleanDev-Fix]

Closing this draft because issue #542 has already been completed by merged PR #567.