docs(governance): Policy Registry sync and versioning guidance

[bokelley]

Closes #3140

Adds the sync/versioning documentation requested by the governance working group, distributed across the files where each topic naturally belongs rather than as a single new section in policy-registry.mdx (per expert review — the five topics split cleanly across audience lines).

What changed

docs/governance/policy-registry.mdx

• Expands "Temporal enforcement" with a worked effective_date example (informational info-severity findings before the date, blocking enforcement after) — consistent with the existing table's "reported at info severity" phrasing
• Adds requires_human_review is not gated by effective_date — the human-review mandate fires as soon as a policy is resolved, not on the enforcement date
• Adds sunset detection guidance: pull-based wall-clock evaluation per check_governance call; no push notification; inspect resolved_policies / policies_evaluated to confirm active set
• Cross-links "Version tracking" bullet to specification#version-pinning

docs/governance/campaign/specification.mdx

• Bug fix: custom_policies and shared_exclusions examples were plain strings; the schema (sync-plans-request.json) defines both as PolicyEntry[] requiring policy_id, enforcement, and policy text. Schema-invalid examples would fail CI JSON validation if they carried $schema refs.
• Adds "Version pinning" subsection under "Policy registry" explaining the policy-ref version field, the protocol-silence caveat on when unversioned plan-level policy_ids re-resolve, and a stability note linking back to the existing "under development" caveat
• Adds additive-only <Warning> block in "Policy resolution" section — quoted from the schema description in policy-entry.json

docs/governance/campaign/tasks/sync_plans.mdx

• Same custom_policies plain-string bug fix in the request example
• Updates custom_policies and portfolio.shared_exclusions field table descriptions to reference PolicyEntry shape and link to policy resolution

Non-breaking justification

Docs-only changes across three MDX files. No schema modifications. No URL changes. Changeset is --empty (no protocol impact). sync_plans surface is x-status: experimental.

Pre-PR review

• code-reviewer: approved — 1 nit (anchor link #policy-registry → #version-pinning, fixed before merge); schema-mismatch fix confirmed correct
• ad-tech-protocol-expert: approved with 2 blockers fixed — (1) "must-level violations" language corrected to "blocking severity" to avoid conflating enforcement level with finding severity; (2) "de-resolved by a plan amendment" replaced with "no longer matched after a plan re-sync" to avoid naming a nonexistent protocol concept

Session: https://claude.ai/code/session_01XuqvXPFRgFRL9tWA9drHCM

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Generated by Claude Code

[bokelley]

Superseded by two PRs:

• docs(governance): policy registry sync and versioning operational guide #3187 (merged) covers the sync/versioning conceptual content from Document Policy Registry sync pattern: version pinning, change handling, effective_date transitions #3140 via a new sibling page (docs/governance/policy-registry-sync.mdx).
• docs(governance): fix schema-invalid custom_policies/shared_exclusions examples #3202 cherry-picks the schema-validity fix from this PR (the real custom_policies/shared_exclusions plain-string → PolicyEntry correction plus the additive-only Warning block).

Closing as superseded — both pieces of the original scope are landing in the two PRs above.