Add configurable session cost budget with guardrail events

[byapparov]

Phase 2 — Security-critical observability

Runaway agents have no circuit breaker. Add configurable session cost threshold.

• Emit cost_warning when approaching limit
• Emit cost_exceeded when hit
• Optionally halt execution on exceed
• Configurable via settings or CLI flag

[Keesan12]

For session caps, the thing I’d avoid is exposing only “budget exceeded” as an event.

Operators usually need three layers:

• the configured cap
• the live burn state while the session is running
• the terminal receipt after the cap or halt fires

That last one matters because otherwise you know the run stopped but not whether it stopped after a useful checkpoint, mid-retry, or after an avoidable failure. We’ve found the receipt is what turns a cap from a safety brake into something you can resume and audit cleanly.