[FEATURE]: Add configurable spending limit / "Kill Switch" per session #4559

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Describe the enhancement you want to request

Is your feature request related to a problem? Please describe.
Currently, there is no built-in mechanism in opencode to stop the agent based on accrued costs. This poses a significant risk: if the agent enters a loop, hallucinates, or performs a very long task without supervision, it can rapidly drain API credits without the user noticing until it's too late.

Describe the solution you'd like
I would like to be able to set a hard monetary limit (e.g., "max cost per session").
This could be implemented via:

1. Configuration: A key in .opencode.json (e.g., "max_session_cost": 2.0).
2. Environment Variable: A standard variable like OPENCODE_MAX_COST=2.0 that sets a global cap for any session running in that environment.
3. Behavior: If the session cost exceeds this limit, the agent should immediately pause or terminate execution and prompt the user for confirmation to proceed.

Additional context
This is a crucial safety feature for autonomous agents to prevent "runaway" billing.
Other tools (like Kilo Code) already offer similar cost-control mechanisms to protect users.