docs: prepare AdCP 3.2 beta release

[bokelley]

Summary

• add the AdCP 3.2 beta program overview, release notes, and 3.1 → 3.2 migration guide
• document the beta.0 → SDK releases → beta.1 convergence cycle and the evidence required before calling beta.1 SDK-backed
• link the beta program to live GitHub milestone views so open candidates stay current without copying a stale issue list
• harden the beta cut runbook, prerelease version/status guidance, SDK support matrix, and docs navigation tests
• remove seven non-protocol/empty changesets while retaining one scoped protocol changeset required for the normative release documentation

Why

The ecosystem needs a single, explicit path for evaluating 3.2 before SDKs are available, tracking work still under consideration, migrating known breaking behavior, and converging on beta.1 after the SDK wave. This makes the prerelease sequence and readiness evidence discoverable before beta.0 ships.

Ecosystem impact

• beta.0 is the protocol-first integration target
• SDKs are cut against beta.0 next
• beta.1 can include protocol corrections discovered during SDK implementation
• SDKs are refreshed against the exact beta.1 bundle before beta.1 is described as SDK-backed
• the migration guide calls out request-signing encoding changes and the unchanged webhook v1 encoding
• milestone links show live open issues, open PRs, and completed work; milestone membership is explicitly a candidate signal, not a release commitment

Expert review

• protocol review: clean after clarifying manifests, beta.1 convergence, signing migration, and end-to-end integration evidence
• documentation review: clean after tightening SDK claims, prerelease status, and creative-delivery replay evidence
• code/release review: clean after making the runbook portable and aligning the pending-beta legend and changeset policy

Validation

• npm run test:docs-nav
• npm run test:owned-links
• npm run test:release-docs-nav
• node scripts/check-changeset-protocol-scope.cjs origin/main
• npx --yes @changesets/cli@^2.31.0 status --since=origin/main
• npm run test:release-workflow
• npm run typecheck
• precommit suite: 438 test files passed; 6,219 tests passed; 30 skipped
• git diff --check origin/main...HEAD

[aao-secretariat]

Ladon verdict: Escalate to human review

Escalate — gated paths touched without required approval.

This PR touches .agents/** files (.agents/playbook.md, .agents/shortcuts/cut-beta.md), which fall under a hard, deterministic approval gate (gated_paths: true). The current review_decision is REVIEW_REQUIRED, not APPROVED, so per decision rule row 2 the gate is not satisfied and the outcome must be escalate rather than approve.

The reviewer found no Critical/High/Medium findings — this is a clean docs-and-runbook PR standing up the AdCP 3.2 beta program (new 3-2-beta / whats-new / migration pages, versions/versioning/release-notes updates, docs.json nav, cut-beta runbook, nav test). Internal links/anchors resolve, version strings are consistent, changeset is present and patch-scoped, and deleted changesets are all non-protocol. No static/schemas/source/**, task defs, or dist artifacts were touched, so there is no wire drift or artifact-immutability risk.

Escalation reasons

• Modifies gated path .agents/playbook.md — requires human/CODEOWNERS approval (review_decision is REVIEW_REQUIRED, not APPROVED).
• Modifies gated path .agents/shortcuts/cut-beta.md — same gate.

Once a human/CODEOWNERS approval lands (review_decision → APPROVED), this clean diff can fall through to approve.

Why human review

• Modifies gated path .agents/playbook.md — hard approval gate, review_decision is REVIEW_REQUIRED (not APPROVED); human/CODEOWNERS review required.
• Modifies gated path .agents/shortcuts/cut-beta.md — same gated-paths approval gate.
• This PR touches a path under a hard, non-overridable approval gate (.agents/playbook.md (modified) matches .agents/**; .agents/shortcuts/cut-beta.md (modified) matches .agents/**) and the current GitHub review decision is 'REVIEW_REQUIRED', not APPROVED. This is a hard gate enforced in code — Ladon cannot auto-approve until a human/CODEOWNERS approval is recorded, regardless of how clean the diff is.