Add contributor agreement and privacy, terms, and accessibility governance

[limxiy]

Summary

Adds a contributor agreement overview and three compliance governance documents,
all self-contained inside Governance/. Each is a versioned policy with an
owner, review cadence, and change log, consistent with the existing Governance/
documents.

File	Covers
Governance/community/CONTRIBUTOR_AGREEMENT.md	What contributors agree to, the DCO sign-off mechanism, and where records are kept.
Governance/compliance/PRIVACY_POLICY_GOVERNANCE.md	Who owns the privacy policy, the update and review process, and versioning.
Governance/compliance/TERMS_OF_SERVICE_GOVERNANCE.md	Who owns the terms, the update and approval process, and the user-notification rule.
Governance/compliance/ACCESSIBILITY_STATEMENT.md	The stated conformance level, the feedback channel, and the review cadence.

Scope

• Documentation only. No TypeScript, configuration, schema, or dependency is
  touched.
• All changes live under Governance/; nothing outside Governance/ is modified.
• Governance/ is outside the Jest root and the TypeScript build, so no test,
  build step, or runtime path is affected.

Notes

• Regression tests are not applicable: this is a documentation-only change with
  no executable code.
• Cross-references point at existing documents (policies/DCO.md,
  policies/CONTRIBUTOR_SIGNOFF.md, domains/, compliance/DPA_TEMPLATE.md,
  SECURITY_POLICY.md, and others).

Closes #1873
Closes #1874
Closes #1875
Closes #1876

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