fix(insane-review): make dependency onboarding work on externally managed Python

[devswha]

Problem

python3 pack_and_ask.py --check-env --install does not complete the documented click-only onboarding on a clean Ubuntu 24.04 host.

Observed on 2026-09-01 while running /omg:insane-review:

1. /usr/bin/python3 had no pip, so both install attempts failed with No module named pip.
2. After installing python3-pip, the normal install command was rejected by PEP 668 (externally-managed-environment).
3. The review could proceed only after an out-of-band system operation and python3 -m pip install --user --break-system-packages pyperclip playwright.

Current implementation at plugins/oh-my-gjc/bin/pack_and_ask.py:897-903 calls:

subprocess.run([sys.executable, "-m", "pip", "install", pip])

It does not check the return code, distinguish missing pip from PEP 668, or provide a reusable isolated environment. The following environment report therefore still says deps=missing after --install, while the command contract says GJC should finish blocked onboarding steps for the user.

Expected behavior

• --check-env --install checks every installer return code and never reports an attempted installation as successful.
• On PEP 668 systems, use a durable per-user venv (preferred) or another explicitly justified isolated mechanism; do not silently modify system Python.
• If python3-venv/pip itself is unavailable, emit a machine-readable status that lets the command ask once and install the OS prerequisite through the selected package-manager path.
• A later --ensure-env automatically reuses the environment without asking again.
• Add an Ubuntu 24.04 regression fixture covering missing pip and externally managed Python.

Evidence

The browser/model path itself succeeded after dependencies were installed: 5.6 Sol · 최대 was verified and a 419-line response was harvested. This issue is limited to environment onboarding, not review correctness.