* 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.
Adds detect_install_method() to identify nixos/homebrew/git/pip installs,
and recommended_update_command_for_method() to return the right upgrade command
for each method. Updates recommended_update_command() to use these for pip-installed
instances (no .git dir, not managed).
