fix(agent-model): pre-landing review fixes

- a broken preferences.yaml no longer blocks --request or config.yaml
- CRLF, a byte-order mark and a trailing comment still read the value
- never echo raw control or bidi bytes from an unvalidated source
- cap the bytes actually read, not only the stat size
- family names and host are case-insensitive in gstack-config too
- list passes the platform through to the agent block
- a wrapper inside the checkout names no preferences file
- docs: --check never writes the skill folder

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

docs/model-policy.md

@@ -157,7 +157,7 @@ over. Keep the preference in `preferences.yaml` next to the gstack skill's
 SKILL.md in your Drive: one `agent_model: <value>` line, with `#` comments
 allowed (any other key is ignored with a warning). The session start puts the
 file's path in the launch file as `GSTACK_PREFERENCES_FILE` and gstack reads it
-where it is: nothing is copied, and `--check` writes nothing. Agents write that
+where it is: nothing is copied, and `--check` never writes the skill folder. Agents write that
 file only when you ask them to remember a preference. To change or clear it for
 good, edit or delete that line; `gstack-config unset agent_model` clears this
 machine only. Copying a newer `capy/skill/gstack/` over the skill folder leaves