Fix #393: enforce accurate timeout budgets in agent chains

[yxlyx]

Summary

• enforce exact per-step timeout budgets in run_task chains so sub-second remainder does not get rounded up into an extra second of wait time
• keep timed-out worker cleanup safe in runChainStep and expose timeout_seconds on run_explorer and run_task
• add timeout-focused tests for budget expiry and timeout payload reporting

Testing

• zig fmt --check src/tools.zig
• zig build test (blocked locally by the current Zig/macOS linker environment: unresolved Darwin libc symbols before project code runs)

Closes #393

[yxlyx]

Superseded by #402, which reapplies the remaining timeout-budget work onto current main after #400.