docs(wechat): use agent-reach interpreter for miku_ai search example (#188)

miku_ai is installed inside the agent-reach Python environment (pipx venv
or user venv). Using bare 'python3' fails when agent-reach was installed
via pipx, because system python3 cannot import miku_ai.

Fix: detect the correct interpreter at runtime via:
  $(python3 -c "import agent_reach, sys; print(sys.executable)")

This resolves the interpreter transparently for pipx, venv, and plain pip
installs without hardcoding paths.

Closes #187

Co-authored-by: Panniantong <panniantong@users.noreply.github.com>