epic(observability): cost telemetry per governed decision

[williamzujkowski]

Narrative

Multi-voter consensus (7 voters) and 5-voter pr_review multiply spend per gated decision. Instrument per-voter, per-strategy, per-decision cost; surface it in weather_report and the strategy-manifest cost profiles (Epic C) so the MetaOrchestrator can route cost-aware via the existing BudgetRouter. Output: a documented answer to "what does a governed decision cost," per gate type.

Grounding (Phase 0 recon)

• Cost plumbing exists in parts: NEXUS_BILLING_MODE (plan zeroes cost in scoring; api keeps cost-aware routing), BudgetRouter is stage 1 of the routing chain, model pricing lives in the registry (in-tree-data.ts), token usage is recorded on outcomes (tokensUsed on ExecutionOutcome/TaskOutcome). What's missing is the AGGREGATION: per-decision rollup across a panel's N voter calls, attributed to the gate/strategy that incurred it.
• Manifest cost profiles (feat(orchestration): versioned strategy-manifest schema #3834) carry placeholders until this epic populates them.
• Related: Revisit batch-submission mode when api-billing volume OR rate-limit throughput pain is demonstrated #2699 (batch-submission when billing pain materializes) — cost data here is the trigger evidence for that decision.

Children

• feat(observability): per-voter, per-decision cost aggregation on consensus + pr_review #3855 per-voter per-decision cost aggregation
• feat(observability): surface decision costs in weather_report + populate manifest cost profiles #3856 weather_report + manifest cost profiles
• docs(observability): 'what does a governed decision cost' — per gate type, with real numbers #3857 the governed-decision cost doc

Dependency notes

M4; depends on Epic C (manifest cost-profile field). Feeds Epic F's ledger cost column and Epic H's positioning honesty.

Definition of done

• A consensus_vote / pr_review / dev-pipeline run can answer "what did this decision cost" from recorded data
• weather_report surfaces per-gate-type cost; manifests carry measured profiles
• The "cost of a governed decision" doc is published with real numbers