Hook output: support native IDE notifications

Summary

Hooks (PreToolUse, PostToolUse, UserPromptSubmit, etc.) can return additionalContext and permissionDecision via JSON stdout. There's no way for a hook to trigger a native IDE notification (VS Code window.showInformationMessage / showWarningMessage / showErrorMessage).

Use Case

Context pressure monitoring: a UserPromptSubmit hook computes context window usage % and wants to show the user a visible notification at threshold crossings (25%, 50%, 80%). Currently the only option is additionalContext which injects text the model sees but the user doesn't see directly.

Proposed API

New hook output field notification:

{
  "notification": {
    "message": "Context at 50% — consider writing CONTINUITY.md",
    "severity": "warning"
  },
  "additionalContext": "CONTEXT PRESSURE: YELLOW (50%)..."
}

severity values: "info" | "warning" | "error" mapping to the VS Code notification API.

Current Workaround

Using a separate VS Code extension that watches a cache file written by the hook. Works but adds an extension dependency for what could be a built-in hook capability.

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