Guard the remaining harness manifests against a dead node runtime

Bugbot caught the Codex plugin builder still invoking node directly, and the
same reasoning covers GitHub Copilot and Grok Build: all three shipped the
exact failure this branch exists to stop, and sat visibly inconsistent with
their guarded siblings.

Route them through guardedNode with no notice, matching Codex and Cursor.
GitHub gains a second property from it: outside a git repository
`$(git rev-parse --show-toplevel)` expands to nothing, so the old command
handed node a path that could not exist and failed the turn. The file test
now short-circuits that to exit 0.

Every builder carries the probe; only the two Claude manifests carry the
notice, which is the only harness whose response shape is confirmed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/hooks/impeccable.json

@@ -5,7 +5,7 @@
       {
         "type": "command",
         "matcher": "edit|create|apply_patch",
-        "bash": "node \"$(git rev-parse --show-toplevel)/.github/skills/impeccable/scripts/hook.mjs\"",
+        "bash": "[ ! -f \"$(git rev-parse --show-toplevel)/.github/skills/impeccable/scripts/hook.mjs\" ] || ! node -e \"import('fs')\" 2>/dev/null || node \"$(git rev-parse --show-toplevel)/.github/skills/impeccable/scripts/hook.mjs\"",
         "timeoutSec": 5
       }
     ]