fix(ha-git-backup): pin core.sshCommand + repo-local known_hosts in install.sh

The nightly HA sync runs via shell_command in the Core container, not the SSH
add-on where install ran. Core has an empty known_hosts, so git push failed
"Host key verification failed" — the commit was made locally but never reached
GitHub. install.sh now seeds a repo-local known_hosts (ssh-keyscan) and pins
git config core.sshCommand so every context (add-on, Core, cron) uses the same
working ssh. Also drops a stray `.git-sync` arg from the pre-commit chmod line.

Records LESSON-06 + a symptom-table row in SKILL.md and SKILL.ru.md; CHANGELOG
Fixed entry. Field-validated on a live Home Assistant.

CHANGELOG.md

@@ -19,6 +19,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   (e.g. M-0073 → HDL, M-00B6 → Ekinex S.p.A.) instead of relying on a hardcoded id map that can
   never be complete; the static map remains as fallback.
 
+### Fixed
+
+- **`skills/ha-git-backup`**: `install.sh` now pins `core.sshCommand` + a repo-local `known_hosts`
+  so the nightly sync pushes from HA's Core container (was failing `Host key verification failed`).
+
 ## [0.8.0] — 2026-07-07
 
 **Lessons from a second field project (a different integrator naming school):** positional