[FEATURE]: Add cancellation support for command.execute.before hook #18554

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Summary

Plugins that intercept commands via command.execute.before have no way to cancel the subsequent prompt() call without throwing an error. This causes a 500 response on the web app (error toast + input restoration), even though the plugin handled the command successfully.

Problem

The command.execute.before hook signature returns Promise<void>. There's no way to signal that the command was fully handled and prompt() should be skipped.

The only way to prevent the LLM call is to throw, which propagates through Plugin.trigger() → SessionPrompt.command() → route handler → onError.

TUI: Unaffected (SDK uses throwOnError: false + fire-and-forget).
Web app: Shows "command failed" error toast and restores the input text.

Suggested fix

Add a cancelled property to the hook output:

// In SessionPrompt.command() (prompt.ts ~line 1895):
const output = { parts, cancelled: false }
await Plugin.trigger("command.execute.before", { ... }, output)
if (output.cancelled) return

const result = await prompt({ ... })

Plugin usage:

"command.execute.before": async (input, output) => {
  if (input.command === "my-command") {
    // Handle the command...
    output.cancelled = true // skip prompt()
  }
}

This would be a minimal, backwards-compatible change, existing plugins that don't set cancelled behave identically.