[Beta.2 continuous cleanup] Consolidate residual repository and governance obligations

[SisyphusZheng]

Part of #1322/#1155 under ADR-0152 / #1341. This issue coordinates only residual obligations; it must not duplicate the checklists in #1328-#1334.

Work

• [B2] Replace generic self-built governance checks with mature OSS #1156 owns mature-tool offload; [Beta.2 continuous cleanup] Freeze semantic ownership and contribution governance #1188 owns contributor/governance documentation convergence; [Beta.2 continuous cleanup] Externalize immutable media and reduce repository weight #1189 owns repository weight and immutable media.
• Cleanup begins with Beta.2.1 replacements and continues through Beta.2.2; Beta.2.3 closes the remainder.
• Remove operational-history dependencies and redundant truth owners, preserving required behavior and evidence. Do not create another checker to reconcile duplicate ledgers.
• Keep [1.0 Alpha] Freeze exact-SHA and immutable-evidence release contract #1187's remaining release provenance/evidence automation in 1.0 Alpha; existing protected release rules remain required meanwhile.

Exit

Residual Beta-owned duties are explicitly assigned, completed with evidence, and not duplicated by this umbrella. Record reduction in scripts/tasks/owners and justified survivors. Broader application/security/provenance qualification remains in the Alpha/RC gates; no feature completion follows from changing milestones.