Beginner-friendly post-install guide: detect & install platform deps automatically

[GGzili]

Summary

When a first-time user runs the official one-line installer and walks
through assets/configure_mykey.py, the wizard currently prints a hint
like:

  💡 提示：你需要安装以下依赖以使消息平台正常工作:
     pip install pycryptodome qrcode requests

…but the user still has to copy/paste it, find the right shell, and
re-run the wizard if the platform needs an interactive QR login (e.g.
WeChat iLink). New users repeatedly hit:

  ⚠ 无法导入 WxBotClient: No module named 'qrcode'

This PR closes that gap.

What changes

1. scripts/post_install.py (new, stdlib-only)

   • Reads the freshly written mykey.py.
   • Detects which IM platforms are enabled (WeChat iLink / Telegram /
     QQ / Feishu / WeCom / DingTalk).
   • For each enabled platform, checks whether its Python packages are
     importable.
   • Offers to install missing packages with uv pip (preferred) or
     python -m pip.
   • For platforms that need interactive QR login, after installing
     deps, offers to re-launch configure_mykey.py so the user can
     scan the QR right away.
   • If no platform is enabled yet, asks the user whether they
     want to enable one and re-launches the wizard.
   • Degrades gracefully when stdin is non-interactive: prints the
     missing deps and exits 0. Never breaks the installer.

2. assets/configure_mykey.py (small change)

   • When deps are listed at the end, hand off to
     scripts/post_install.py if it exists and stdin is interactive.
   • Falls back to the existing copy/paste hint otherwise.

3. .github/workflows/ci.yml (new)

   • lint-python: compileall on scripts/ and assets/, plus
     ruff (warn-only).
   • lint-shell: shellcheck any scripts/*.sh shipped in the
     repo.
   • test-post-install: matrix on Ubuntu/macOS/Windows × Python
     3.11/3.12. Runs post_install.py against (a) empty mykey.py,
     (b) a mock Telegram config without the telegram package
     installed, asserting the script exits 0 and is non-interactive
     friendly.
   • smoke-developer-install: uv pip install -e . and
     import agent_loop to catch packaging regressions.

Why a separate Python script (and not just shell-side logic)?

• The official one-line installer is split between a bash and a
  PowerShell variant, both hosted on a VPS. Putting the logic in a
  stdlib-only Python script keeps it cross-platform, in-repo, and
  reviewable.
• Once this lands, both VPS scripts only need a one-liner:
  python scripts/post_install.py — see the companion issue I'll
  file for the VPS-side change.

Out of scope (intentionally)

• Does not change the platform detection regex set in
  configure_mykey.py; we read the same identifiers it writes.
• Does not introduce any new runtime dependencies.
• Does not auto-install LLM extras (still scoped to IM platforms,
  which are the noisy ones for new users).

Test notes

• Local: ran wizard with no platform → got "Enable a platform now?"
  prompt; with WeChat iLink enabled → got missing-dep prompt, then
  QR-rerun prompt.
• CI matrix runs both empty and mock-config flows non-interactively.