docs(compliance): add sales-specialism decision tree

[bokelley]

Closes #4038

Sell-side adopters had no single source of truth for choosing a sales-* specialism — the only path was to read every storyboard narrative and triangulate. A wrong claim wastes the adopter's first compliance run by grading them against storyboards their architecture doesn't support.

This PR adds a ## Choosing a sales specialism three-step decision tree (<Steps>) to compliance-catalog.mdx, placed after ### brand and before ## How to claim. The tree resolves adopters to sales-broadcast-tv, sales-catalog-driven, sales-social, sales-guaranteed, or sales-non-guaranteed; covers the multi-specialism case (hybrid guaranteed + auction platforms); and explains the media_buy.supports_proposals capability flag with jsonc wire examples. It also corrects sales-proposal-mode status from stable → deprecated in four files (compliance-catalog.mdx, learning/specialist/media-buy.mdx, learning/tracks/publisher.mdx, build-an-agent.mdx), and fixes a pre-existing creative-hosting → creative-ad-server typo in get-test-ready.mdx.

Non-breaking justification: adds a new doc section and corrects status labels/example JSON; no schema, storyboard, or enum values changed. Existing clients unaffected.

Pre-PR review:

• code-reviewer: approved — fixed two blockers (commented json fences → jsonc; two learning-docs files showed sales-proposal-mode as stable)
• docs-expert: approved — fixed two additional blockers (Step 1 caveat implied broadcast-only sellers must also claim sales-guaranteed; build-an-agent.mdx Tip block still said "proposal mode"); also moved section to correct ##-level position after all ### specialism subsections

Nits (not blocking, left for human reviewer):

• Step 3 title Set media_buy.supports_proposals (sales-guaranteed only) mixes case conventions — can be polished to Configure media_buy.supports_proposals

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See #3121
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Session: https://claude.ai/code/session_017tXWUzFaMeS77vRFinT25W