docs: Media Buy Lifecycle page with state diagram and PG variation

[bokelley]

Context

Addie weekly insights (Apr 11–18): ~75 technical-implementation questions covering media buy lifecycle, creative sync timing, PG deals, and SSP integration patterns. No single canonical page to point builders at.

Problem

Lifecycle mechanics are documented piecemeal across task reference pages. Builders asking 'what's the sequence?' or 'how does PG differ?' get re-explanations from Addie instead of a canonical doc.

Acceptance criteria

New page at docs.adcontextprotocol.org covering:

• State diagram: active, pending_manual, pending_creatives, pending_start (+ terminal states).
• Sequenced flow diagram: get_products → create_media_buy → sync_creatives → delivery.
• Guaranteed / PG deal variation: proposal / IO acceptance path, where it diverges from the standard flow.
• Creative sync timing: when creatives are required, what happens if they're late.
• Linked from the quickstart and from the sales-agent / buyer-agent reference pages.

Related

• Media Buy Lifecycle: Creative state undefined after buy rejection/cancellation #2254 Media Buy Lifecycle: Creative state undefined after buy rejection/cancellation (this doc should reflect the resolution)
• Creative lifecycle webhooks: formalize state-change notifications outside media buy lifecycle #2261 Creative lifecycle webhooks

Source

Addie conversation insights, Apr 11–18.

[bokelley]

Triage: Execute — drafted PR #3354

New page docs/media-buy/media-buys/lifecycle.mdx with Mermaid sequenced flow (get_products → create_media_buy → sync_creatives → delivery), full state machine diagram (all 7 states + transitions), guaranteed/PG deal IO acceptance path, creative sync timing with creative_deadline hierarchy. Nav entries added in both tabs; linked from quickstart and sales-agent-creative-capabilities.mdx.

PR: #3354

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