Add PocketPay Mobile issue approval readiness checklist

[vickman787]

Summary

• Adds docs/issue-approval-readiness-checklist.md, a short pre-approval gate covering implementation completeness, testing, CI status, acceptance criteria, documentation, and known limitations.
• States explicitly that a merged PR does not guarantee payment approval.
• Links the new checklist from the README and from the CONTRIBUTING.md PR-submission steps, alongside the existing Evaluation Readiness Checklist and Contributor Self-Assessment.

Closes #493

Test Plan and Evidence

Documentation-only change. Static review performed:

• Confirmed the new doc renders correctly as Markdown and all internal links (contributor-self-assessment.md, evaluation-readiness-checklist.md, test-first-contribution-guide.md, ci-troubleshooting.md) resolve to existing files in docs/.
• Confirmed the new README and CONTRIBUTING.md links point to the correct new file path.
• No source, test, or CI configuration was touched, so no automated tests apply.

Self-Assessment

• Scope: The change matches the linked issue and contains no unrelated or unfinished work.
• Tests: Documentation-only change; no runtime tests are applicable. Verified via manual link/render review above.
• CI: No code changes; CI checks (test/typecheck/lint) are unaffected by this PR.
• Documentation: README.md and CONTRIBUTING.md updated to link the new checklist.
• Known limitations: None known — this checklist intentionally stays short and cross-links to the more detailed Evaluation Readiness Checklist and Contributor Self-Assessment rather than duplicating them.
• Acceptance criteria: See table below.

CI Status

No CI-relevant files changed.

Documentation

• Added docs/issue-approval-readiness-checklist.md.
• Updated README.md to link the new checklist.
• Updated CONTRIBUTING.md PR-submission section to reference the new checklist before the full Evaluation Readiness Checklist.

Known Limitations

None known.

Acceptance Criteria

Acceptance criterion	Implementation or evidence	Status
Approval readiness checklist is added	docs/issue-approval-readiness-checklist.md	Pass
Checklist states that merged PRs are still subject to evaluation	Callout at top of the new doc	Pass
Testing expectations are included	Section 2 of the new doc	Pass
CI status expectations are included	Section 3 of the new doc	Pass
Acceptance criteria review is included	Section 4 of the new doc	Pass
README links to the checklist	README.md Documentation list	Pass

Screenshots or Recordings

Not applicable — documentation-only change with no visible UI or app behavior change.