feat(config): add install-method stamping + Docker detection (#27843)

* feat(config): add install-method stamping + Docker detection

Dockerfile stamps "docker", install.sh stamps "git", and cmd_postinstall
stamps "pip" into ~/.hermes/.install_method. detect_install_method() reads
the stamp first, then falls back to managed-system / container / .git
heuristics. Adds Docker upgrade guidance.

Tracking: #27826

* fix(stamp): move Docker stamp to entrypoint, install.sh stamp after print_success

The Dockerfile stamp was overwritten by the VOLUME overlay at container
start. Moving it to entrypoint.sh ensures it persists. The install.sh
stamp now writes after print_success so it only lands on full success.

scripts/install.sh

@@ -1996,6 +1996,8 @@ main() {
     maybe_start_gateway
 
     print_success
+
+    echo "git" > "$HERMES_HOME/.install_method"
 }
 
 if [ -n "$ENSURE_DEPS" ]; then