Wire burn hook-based Claude Code ingest + retire CostTracker

[willwashburn]

Follow-up from burn#7 (shipped in @relayburn/*@0.8.0).

What to do

A. Wire buildClaudeHookSettings into the two spawn sites

Burn now exports a pure spawner primitive from @relayburn/ledger:

import { buildClaudeHookSettings, stamp } from '@relayburn/ledger';

const { sessionId, settings } = buildClaudeHookSettings();
await stamp({ sessionId }, { workflowId, agentId, persona, tier });

spawn('claude', [
  '--session-id', sessionId,
  '--settings', settings,
  ...existingArgs,
]);

Apply the same pattern at both spawn sites:

• packages/sdk/src/workflows/process-spawner.ts:68 (non-interactive agent steps)
• packages/sdk/src/workflows/runner.ts:5887 (interactive agents via relay.spawnPty)

Relay's existing env-var threading (AGENT_NAME, RELAY_API_KEY, AGENT_CHANNELS at runner.ts:5863-5868) is orthogonal — leave it alone.

B. Retire CostTracker

src/cost/tracker.ts writes a wall-clock-based token estimate to ~/.agent-relay/usage.jsonl. Once (A) is in place, burn's hook-fed ledger at ~/.relayburn/ledger.jsonl is the authoritative source for the same data (exact, not estimated). Delete CostTracker and any call sites.

Acceptance

• Both spawn sites pass --session-id + --settings when spawning claude.
• Stamp metadata (workflow, agent, persona, tier) is applied against the pre-assigned UUID before spawn so all turns inherit it.
• CostTracker and ~/.agent-relay/usage.jsonl are removed; any UI that reads the old file now reads from burn.
• End-to-end: spawn an interactive agent, run one tool call, verify a TurnRecord with the stamped enrichment shows up in burn query --session <id>.