fix(tokens): count a subagent's tokens as the session's

[ericleepi314]

The bug

Every token read-out undercounted a run that fanned out.

A child's assistant/message usage was added to the child's own row — the agents overlay, and the Done (3 tool uses · 1.2k tokens) a delegation settles with — and stopped there. The session odometer behind the stats line's tokens: N in / M out, /status, and the status rule's context read-out never heard about it. A turn that delegated most of its work reported only the sliver the parent did itself:

tokens: 1.1k in / 84 out        ← parent only, what it showed
tokens: 42.6k in / 3.1k out     ← what the session actually spent

The stats line has claimed subagent-inclusive totals since it was written (src/lib/sessionStats.ts: "the same accumulators /cost prints, subagents included"). It just wasn't true on this backend.

The fix

onChildSessionEvent folds a registered child's step into usageTotals alongside the child's own counters.

The two halves of the bus are already disjoint — bindAgent routes by session identity, and onChildSessionEvent drops anything the subagent lifecycle never announced as a child — so the step is counted exactly once, and a sibling top-level session in the same process still cannot leak in. There's a test for that second half.

Tests

• bills the child's tokens to the session odometer, not just its own row — parent step + child step (with a cache read) land in one total; fails on main with calls: 1, input: 100.
• ignores a sibling session that was never announced as a child — guards the disjointness the fix relies on.

Plus the coverage for the fold that #23 carried in without it: statsFromUsage, its precedence behind a CostSnapshot, the price-free usage path through message.complete, and the rider a resume replays out of the log.

Verification

npm test — 150 files, 1950 passed / 11 skipped. npm run typecheck and npm run verify:boundary clean.

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