docs: add central evaluation-readiness index page (#122)

[rexx010]

Closes #122

Summary

Adds one central page, docs/evaluation-readiness-index.md, that links every
evaluation-readiness requirement for contributors instead of leaving that
guidance scattered across the README, PR template, testing docs, and CI docs.

What changed

docs/evaluation-readiness-index.md (new file, 68 lines)

New index page with 7 sections, each linking to the existing authoritative doc:

1. Payment Expectations → contributor-payment-guide.md, contributor-evaluation-policy.md, payment-period-conduct.md
2. Testing Standards → testing.md, contributor-test-matrix.md, test-first-contribution-guide.md, testing-evidence-requirement.md
3. CI Guidance → ci-pass-requirements.md, ci-resolution-workflow.md, ci-response-guide.md
4. Acceptance Criteria Audit → acceptance-criteria-audit.md, acceptance-criteria-traceability.md
5. Self-Assessment → self-review-template.md, issue-approval-readiness.md
6. Reviewer Checklist → reviewer-checklist.md
7. Communication Policy → payment-period-communication-policy.md
   Plus a "Related References" section linking pr-evidence-checklist.md,
   meaningful-implementation-checklist.md, meaningful-change-threshold.md,
   and low-effort-pr-examples.md. The page links out to each doc rather than
   duplicating content, so it can't go stale relative to the source docs.

README.md (+4 lines, at line 143)

Added a pointer to the new index right under the existing Contributing
heading, so it's visible before a contributor reads further.

CONTRIBUTING.md (+4 lines, at line 5)

Added the same pointer at the top of the file, before the Development
Workflow section, so it's the first thing a contributor sees.

Acceptance criteria mapped

Criterion	Status	Evidence
Evaluation-readiness index page is added	✅	docs/evaluation-readiness-index.md (new, 68 lines)
Payment expectation guide is linked	✅	Index §1, links contributor-payment-guide.md
Testing standard is linked	✅	Index §2, links testing.md + contributor-test-matrix.md
CI guidance is linked	✅	Index §3, links ci-pass-requirements.md + 2 more
Acceptance criteria audit is linked	✅	Index §4, links acceptance-criteria-audit.md + traceability table
Self-assessment and reviewer checklist are linked	✅	Index §5 (self-review) and §6 (reviewer checklist)

Why no code/test changes

This is a documentation-only change (issue type: Documentation). No src/,
tests/, .github/, or package.json changes were needed — the issue's
"Files or Areas Likely Affected" list was a superset of possible touch points,
and only docs/, README.md, and CONTRIBUTING.md were actually required to
satisfy the acceptance criteria.

Testing evidence

Full suite run before the change (baseline, main):

$ npm run test
Test Suites: 11 passed, 11 total
Tests:       117 passed, 117 total
 
$ npm run check
> tsc                         → no errors
> jest --runInBand            → Test Suites: 11 passed, 11 total / Tests: 117 passed, 117 total
> node scripts/check-compat.mjs
  Browser compatibility: bundle resolved without Node-only imports.
  Node compatibility: public SDK entrypoint and signer initialized.

Full suite run after applying this change (branch docs/evaluation-readiness-index-122):

$ npm run test
Test Suites: 11 passed, 11 total
Tests:       117 passed, 117 total
 
$ npm run check
> tsc                         → no errors
> jest --runInBand            → Test Suites: 11 passed, 11 total / Tests: 117 passed, 117 total
> node scripts/check-compat.mjs
  Browser compatibility: bundle resolved without Node-only imports.
  Node compatibility: public SDK entrypoint and signer initialized.

Identical results before and after (117/117 tests, clean build, clean compat
checks) — expected, since this PR only touches Markdown files under docs/
and two root-level .md files.