fix(agent-cli): keep pi-reported generation cost without a resolver or token usage

[ayush-or]

TL;DR

Two gaps in the pi billed-cost path from #128 dropped cost the agent had already reported:

1. resolveBilledCost returned undefined when no GenerationResolver was provided, ignoring reportedGenerationCosts.
2. withBilledCost returned undefined usage when pi sent a cost but no token usage, so ori-solver fell back to ZERO_USAGE and recorded totalCost: 0.

Both were flagged by Devin Review on the vendored copy of #129 downstream; they have to land here first.

What changed?

• resolveBilledCost sums the reported costs when no resolver is present (still undefined if none were reported).
• withBilledCost builds a zero-token usage carrying totalCost when usage is missing but a cost resolved; existing usage is unchanged apart from totalCost.
• Regression tests in runner.test.ts for both.

Before: resolveBilledCost(["a"], new Map([["a", 0.5]])) with no resolver → undefined; cost event + empty usage → totalCost: 0.
After: 0.5 in both cases.

How to test

• bun test src/benchmarks: 1194 pass, 0 fail (local). format:check, check, typecheck pass locally.
• Before the fix I reproduced both with throwaway tests against parseRun / resolveBilledCost on main.

Reviewer focus

The real runner always provides a resolver (run-by-id.ts), so fix 1 only affects callers without one; fix 2 needs pi to omit token usage while the fetch hook still captured a cost.