docs(hermes): describe the directory-plugin install the bootstrap performs

[nir-singher-band]

Why

Follow-up to #21. hermes/README.md and hermes/TESTING.md still described the pip install into the gateway Python that bootstrap.sh stopped doing when it moved to the repo's install.sh (directory plugin). Two were actively broken instructions:

• TESTING.md told you to swap in uv pip install --python "$hermes_python" -e … — $hermes_python is defined by no script in the repo any more.
• The Part 2 verification snippet resolved the skill via import hermes_band_platform, which a directory install cannot satisfy: the plugin is copied to $HERMES_HOME/plugins/band, not installed into the gateway venv.

What changed

Both guides now state what the install actually does, per upstream install.sh:

• stage the plugin into $HERMES_HOME/plugins/band, resolve band-sdk into $HERMES_HOME/band-libs with the gateway interpreter, write nothing to the gateway venv, enable band;
• resolve the skill by path and the interpreter with the plugin's own gateway_python.py --print — the way install.sh and bootstrap.sh both do it.

Constraints that follow, now recorded:

• a directory plugin is a copy, so local plugin edits need install.sh re-run — there is no editable install;
• a pip copy of hermes-band overrides a directory install in Hermes's loader, so install.sh refuses to run while one is present (BAND_UNINSTALL_PIP=1 removes it);
• verify_install.py lists package_importable/entry_point under missing on a correct directory install, so blocking is the field to read. Checklist and troubleshooting updated accordingly.
• uv added to the TESTING prereqs — install.sh hard-requires it for uv pip install --target.

The setup skill is addressed as band:add-band, matching the hand-off bootstrap.sh runs. The previous hermes /add-band had nothing behind it: the band plugin registers no slash command (no register_slash_command in the plugin), and bootstrap.sh documents that plugin skills never enter the flat ~/.hermes/skills tree.

Docs only — no change to bootstrap.sh or scripts/.

Test plan

• python3 scripts/check.py
• pytest tests/ -q (17 passed)
• The new Part 2 snippet executed against the real upstream gateway_python.py in an installed-layout tree: resolves ~/.hermes/hermes-agent/venv/bin/python, Python 3.11.15, hermes_cli importable — matching the installer's 3.11–3.13 + hermes_cli contract.
• Every claim checked against upstream install.sh / verify_install.py rather than the previous prose.