fix(claude): forward the billed dollar receipt off the result line

[drewstone]

Problem

ClaudeStreamResult declares total_cost_usd (src/backends/claude.ts:74), but the terminal yield sent only { finish_reason, usage, internal_session_id }. total_cost_usd is a sibling of usage, not a member, so the one genuinely provider-billed number the bridge receives never left the backend.

Measured consequence downstream: across 292 fleet runs, spentTotal.usd > 0 on zero runs. The dollar channel is dead, and this is one of the two reasons.

Change

Forward the figure through the lane the bridge already uses for a known cost (src/usage.ts:59-67, src/streaming/sse.ts:108-121):

Case	What travels
total_cost_usd present, finite, >= 0	cost, cost_known: true, cost_provenance: provider-receipt, cost_scope: total
absent	cost_known: false, no amount
negative / NaN / Infinity	cost_known: false, no amount

cost_scope: total is correct because Anthropic's figure covers the whole claude -p invocation, including every internal model call the harness made.

It is a real provider receipt, so it goes in the receipt lane, not the estimate lane. The catalog-estimate lane is untouched: Pi keeps costKnown: false by contract (src/backends/types.ts:138) because neither Pi nor the proxy reports billed dollars.

A run reporting no figure sends no dollar amount at all. A zero would read as a measured free turn.

Proof

New tests/claude-cost-receipt.test.ts (5 tests). 4 of the 5 fail without the source change — verified by stashing src/backends/claude.ts and re-running:

- "cost": 0.0412,
- "cost_known": true,
- "cost_provenance": "provider-receipt",
- "cost_scope": "total",
+ "cost_known": false,

 Test Files  1 failed (1)
      Tests  4 failed | 1 passed (5)

The 5th (catalog estimate stays out of the receipt lane) passes both before and after — it is the regression guard for the lane this change must not touch.

Full suite, after:

 Test Files  45 passed | 1 skipped (46)
      Tests  815 passed | 8 skipped (823)
   Duration  8.72s

pnpm typecheck clean.

tests/pi-backend.test.ts gains an explicit guard that no Pi delta carries cost_known: true or a cost amount.

Docs

Two README bullets: the two-lane dollar contract, and what the claude harness now reports.