fix(compliance): chain proposal_id between storyboard steps in proposal_finalize + sales-proposal-mode

[bokelley]

Closes #4086

Both media_buy_seller/proposal_finalize and sales-proposal-mode storyboards were sending the literal string "balanced_reach_q2" as proposal_id in their refine/finalize/accept steps. Proposal-mode sellers with runtime-generated IDs (UUIDs, DB rowids) received 404s from their upstreams on those steps. The fix adds context_outputs on the brief_with_proposals step to capture the seller-minted proposal_id, then substitutes $context.proposal_id in every subsequent step that references it. Catalog-mode sellers with stable IDs are unaffected — the chained value equals the old literal for any seller whose brief step returns the same ID.

Non-breaking justification: Adds optional context_outputs chaining to existing storyboard steps (additive) and an optional proposal_id property to CreateMediaBuySuccess schema (additive, additionalProperties: true already permitted the field). No existing fields renamed or removed.

Scope note: The protocol expert flagged that sales-proposal-mode/index.yaml carries the identical "balanced_reach_q2" placeholder bug. Both files are fixed in this PR rather than a separate follow-up to avoid the "small follow-up gets lost while the parent ships and re-opens the same file" failure mode.

Open follow-on (not in scope): finalize_proposal/get_products_finalize has prose-only assertions for proposals[0].proposal_status: committed and proposals[0].expires_at. Adding machine-checkable field_value / field_present validations for those would improve coverage — filed separately.

Pre-PR review:

• code-reviewer: approved — YAML shape (name + path on context_outputs, $context.<name> in sample_request) is correct; context_outputs on stateful: false step is valid per schema; one nit (missing blank line before - id: review_refine) fixed before commit
• ad-tech-protocol-expert: approved — chaining approach is protocol-correct; proposals[0] index convention matches corpus; added proposal_id as optional property to CreateMediaBuySuccess to resolve the field_present validation's blocker risk (schema didn't previously declare the field)

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See #3121
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Session: https://claude.ai/code/session_01CvVpewM8Xh3bG49S8QGZoM

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[bokelley]

This PR was opened by the triage routine. See issue #4086 for context.

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Generated by Claude Code