[feature]: Sub-cent remaining budget rounds --max-budget-usd to 0.00

[faisalishfaq2005]

What problem would this solve?

In src/core/claude.ts, the per-step cost cap is passed as inv.maxBudgetUsd.toFixed(2). When the runner's remaining budget is a fraction of a cent (e.g. $0.004 left of a $2 ceiling), this sends --max-budget-usd 0.00. The runner's own budget.exhausted check (spent >= max) only fires between steps, so a step can still be invoked with an effectively-zero or ambiguous cap.

Proposed solution

Decide and document the intended behavior — e.g. round the cap up to 0.01, or treat "less than one cent remaining" as exhausted and skip the step. Add a test in tests/ covering the sub-cent boundary.

Alternatives considered

No response

Scope

• I've checked this isn't already on the roadmap in the README.
• This can't reasonably be expressed as a loop instead of engine code.