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.

docker/entrypoint.sh

@@ -61,6 +61,9 @@ fi
 # --- Running as hermes from here ---
 source "${INSTALL_DIR}/.venv/bin/activate"
 
+# Stamp install method for detect_install_method()
+echo "docker" > "${HERMES_HOME:=/opt/data}/.install_method" 2>/dev/null || true
+
 # Create essential directory structure.  Cache and platform directories
 # (cache/images, cache/audio, platforms/whatsapp, etc.) are created on
 # demand by the application — don't pre-create them here so new installs