feat: extend signals protocol with transaction support

[bokelley]

Summary

• New signal-pricing.json schema: discriminated union on model — cpm (fixed rate) and percent_of_media (percentage of spend, with optional max_cpm hybrid cap)
• New signal-pricing-option.json: wraps pricing_option_id + pricing model; returned in get_signals, referenced in report_usage
• New generic report_usage task (account/report-usage-request.json + response): cross-protocol usage reporting shared across signals, governance, and creative agents; supports batching multiple campaigns per request via per-record buyer_campaign_ref; requires operator_id to characterize billing responsibility
• get_signals updates: accepts optional account_id and buyer_campaign_ref; response replaces single pricing field with pricing_options[]
• activate_signal update: accepts optional account_id and buyer_campaign_ref for transaction context
• signal-filters.json: adds max_percent filter for percent-of-media signals
• Docs: updated signals overview, specification, get_signals task reference, and account management integration guide

Standard transaction flow

sync_accounts → get_signals(account_id, buyer_campaign_ref) → [campaign runs] → report_usage(pricing_option_id, impressions, media_spend)

activate_signal is reserved for custom signals that require explicit DSP-side activation.

Test plan

• All 304 existing tests pass (pre-commit hook verified)
• Schema validation passes (327 schemas, all $refs resolve)
• Example validation passes
• TypeScript typecheck passes
• OpenAPI spec is current
• Mintlify docs validation passes (362 MDX files)

🤖 Generated with Claude Code

[bokelley]

Superseded by #1204, which ports the remaining schema changes (#1201 covered pricing_options on the response but missed the signal-specific pricing models, percent_of_media, and transaction context fields).