fix(install): work on bare servers without PATH hermes or python3-venv

fix(install): work on bare servers without PATH hermes or python3-venv

Found during first install on a fresh Debian 13 server:

- Installer now probes ~/.local/bin/hermes, ~/bin, /usr/local/bin and
  /usr/bin for the hermes CLI instead of relying on PATH alone.
  Non-login SSH shells never read ~/.profile, so "hermes CLI not found"
  fired and memory.provider was left unset.
- New shared Tools/lib/common.sh picks a working Python for all Tools
  scripts: project .venv (only if sqlite-vec + model2vec actually
  import — a failed python3 -m venv on hosts without the python3-venv
  apt package leaves a half-created venv that passes -x checks), then
  uv (bundled with Hermes at ~/.hermes/bin/uv), then the Hermes runtime
  venv. Broken half-venvs are recreated instead of trusted.
- Docs: prerequisites updated (no PATH / python3-venv needed), importer
  fallback and idempotency documented, systemd restart hint including
  the "Gateway already running (PID ...)" stale-lock case.
- AGENTS.org: Tools/lib layout entry and three new pitfalls.