docs: add reviewer evidence checklist (#498)

[Terryodib]

Summary

Adds a maintainer-facing Reviewer Evidence Checklist so reviewers evaluate whether a PR is complete and evaluation-ready against the same evidence every time, instead of merging on a green checkmark or a convincing description alone.

Every existing checklist in the repo is contributor-facing (Self-Review Checklist, Contributor Self-Assessment, Issue Approval Readiness, Evaluation Readiness). This new doc is the mirror image: it tells the reviewer what to confirm on the diff, the test output, and the CI run itself before approving.

Closes #498

What's included

New doc docs/reviewer-evidence-checklist.md, covering:

• Implementation Scope — the diff addresses the linked issue, is complete, and bundles no unrelated churn
• Implementation Quality — real implementation, all UI states handled, no dead code or leftover TODOs
• Test Evidence — happy-path and negative coverage, tests that actually exercise the change, real test output rather than a claim
• CI Status — npm test, npm run typecheck, npm run lint green on the latest commit, plus npm run api:check where the change touches src/types/pocketpay-sdk.d.ts or src/sdk-stub/
• Documentation Impact — affected docs and README references updated
• Acceptance Criteria — each criterion checked individually against the diff and mapped to concrete evidence
• Risk — blast radius, security-sensitive areas, boundary conditions, and residual follow-up work

Plus a "Ready To Approve When" gate and cross-links to the related contributor guidance.

Acceptance criteria

• (a) Reviewer evidence checklist is added — docs/reviewer-evidence-checklist.md
• (b) Implementation scope is checked — Section 1
• (c) Test evidence is checked — Section 3
• (d) CI status is checked — Section 4
• (e) Acceptance criteria are checked — Section 6
• (f) README links to reviewer guidance — new entry under the Documentation section

Notes

Documentation-only change. No application code, tests, or dependencies are touched.