docs: add contributor self-assessment form

[Carlys17]

Closes #164

Description

Add a new structured pre-submission self-assessment form at docs/CONTRIBUTOR_SELF_ASSESSMENT_FORM.md.

The form asks contributors to confirm:

1. Scope (matches issue body, no unrelated changes)
2. Test evidence (commands, results, no-test justification)
3. CI status (pnpm verify, contract tests if applicable)
4. Known limitations (edge cases, thinner coverage, assumptions)
5. Acceptance criteria confirmation (table mapping each criterion to status + evidence)
6. Final assessment decision (Ready for submission / Needs more work / Needs maintainer input)

The form is distinct from the existing CONTRIBUTOR_SELF_REVIEW_TEMPLATE.md (post-implementation checklist). This form is the pre-submission structured assessment used before review or payment-period evaluation.

README docs index is updated with a link to the new form.

Acceptance Criteria Coverage

• Self-assessment form is added
• Scope confirmation is included
• Test evidence is included
• CI status is included
• Known limitations are included
• Acceptance criteria confirmation is included