governance: add contributor offboarding process and role promotion criteria

[Taylor-Kay]

Summary

Resolves the two governance gaps assigned to @Taylor-Kay by adding a
contributor offboarding process and an explicit role promotion criteria
document. All changes are scoped to the Governance/processes/ folder.

Closes #1484
Closes #1483

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Changes

Issue #1484 — Contributor Offboarding Process

Governance/processes/OFFBOARDING.md

• Defines three offboarding triggers: voluntary departure,
  inactivity-triggered (per INACTIVITY.md), and role removal.
• Per-role access revocation: contributors (status note only),
  reviewers (CODEOWNERS removal), maintainers (team + elevated
  permissions removed, individually held secrets rotated within
  24 hours, shared credentials within 5 business days).
• Knowledge handover section: expected for reviewers, required for
  maintainers/leads, with a 10-business-day window.
• Full timeline of every step with owners and deadlines.

Governance/processes/OFFBOARDING.test.ts

• 25 regression tests covering triggers, access revocation, handover
  requirements, and timeline commitments.

Issue #1483 — Role Promotion Criteria

Governance/processes/PROMOTION_CRITERIA.md

• Contributor: 1 merged PR closing an assigned issue, quality gates
  pass, no conduct matter. Granted automatically.
• Reviewer: 5 merged PRs across 2+ areas, 5 substantive reviews, 3
  months active, no gaps > 30 days.
• Maintainer: 15 merged PRs (5 non-trivial), 20 substantive reviews
  (5 cited as influential), 6 months at Reviewer level.
• Nomination and approval follows NOMINATION.md (public issue, 1
  second within 10 business days, decision within 5 business days).
• Evidence requirements with mandatory links; assertions without links
  do not satisfy the requirement.

Governance/processes/PROMOTION_CRITERIA.test.ts

• 34 regression tests pinning per-role thresholds, approval process,
  and evidence requirements.

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Test results

✓ Governance/processes/OFFBOARDING.test.ts        (25 tests)
✓ Governance/processes/PROMOTION_CRITERIA.test.ts (34 tests)

Tests  59 passed (59)

The 3 pre-existing failures in INACTIVITY.test.ts and
BREAKING_CHANGES.test.ts are present on main and are not
introduced or affected by this PR.

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Acceptance criteria

• Governance requirement implemented successfully
• Scope limited to a maximum of two files per issue
• No changes made outside the Governance/ folder
• No regression in existing functionality
• Tests pass and code follows project standards
• Change is documented (Revision History in each file)

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