Creative lifecycle webhooks: formalize state-change notifications outside media buy lifecycle

[bokelley]

Problem

AdCP webhooks today cover sync_creatives task completion and media buy lifecycle events, but not creative state transitions that happen independently of a campaign. Examples buyer agents need to react to:

• Seller pulls an approved creative back to pending_review on policy change
• Seller rejects a creative post-approval
• Seller purges a creative under a retention policy (see retention sub-issue)
• Creative is re-reviewed and the outcome flips

The only relevant language today is sales-agent-creative-capabilities.mdx:182: sellers "SHOULD notify the buyer when a previously approved creative is pulled back for re-review" — no schema, no delivery contract, no subscription model.

Proposed additions

1. Formalize SHOULD → MUST for state-change notifications where the change is not initiated by the buyer.
2. Define event schemas — at minimum:
   • creative.status_changed (old status, new status, reason, initiator)
   • creative.purged (retention-driven or policy-driven)
   • creative.reassignment_required (active assignments affected)
3. Subscription model — buyer-account-level subscription, not per-buy. These events may fire long after any media buy has closed.
4. Delivery semantics — align with existing webhook retry/ack rules in docs/building/implementation/webhooks.mdx.

Questions to resolve

• Should creative.status_changed also fire for buyer-initiated archives (redundant with the sync response), or only for seller-initiated transitions?
• How does this interact with the already-open Publish JSON schemas for update_rights and creative_approval (or clarify webhook-only status) #2253 (update_rights and creative_approval webhook schemas)? Likely the same event family.
• What's the retroactive contract for creatives approved before the webhook is formalized?

Context

Surfaced by #2254. The autonomous-agent use case makes this urgent: a buyer agent orchestrating campaigns across multiple sellers cannot rely on poll-on-every-use to know if a library asset is still valid.

Relates to #2254, #2253, epic lineage #1612-#1616.