opencode-cost-guard monitors your session cost in real time and fires a
  warning message when you approach your budget, then a limit-reached alert
  when you cross it.
  OpenCode has no built-in per-session cost cap. This plugin fills that gap
  with a single line of config.
  **Features**
  - Early warning at a configurable percentage of your budget (default 80%)
  - Hard limit alert in warn or block mode when the threshold is crossed
  - Each alert fires once per session — no spam
  - Per-project config overrides the global one
  - Zero dependencies, one line to install
  **Install**
  Add to `~/.config/opencode/opencode.json`:
  ```json
  { "plugins": ["opencode-cost-guard"] }

  **Configure**
  Drop a cost-guard.config.json in ~/.config/opencode/:
  ```json
  {
    "maxCostUsd": 20.0,
    "warnAtPercent": 80,
    "mode": "warn"
  }

  Set mode to "block" to stop the session when the limit is reached.
  Set COST_GUARD_DEBUG=1 to enable verbose per-event logging.

  Note: The limit is reactive — it fires after a response completes,
  not before. One expensive response can push you over before the plugin
  catches it.