After `hermes update`, a globally-installed agent-browser's npm postinstall
(fixUnixSymlink) re-points the global symlink (e.g. /opt/homebrew/bin/agent-browser)
at our local node_modules binary. The next update wipes node_modules, leaving a
dangling symlink that `which` still reports but exec fails on with exit 127 —
silently breaking every browser tool (#48521).
Root cause is trust-on-presence: shutil.which/Path.exists accept a name that
resolves but won't run. Add hermes_constants.agent_browser_runnable() (resolves
the path + runs --version) and gate all four resolution sites on it:
_find_agent_browser now skips a dead candidate and falls through to the next
working one (extended PATH -> local .bin -> npx), self-healing the dangling link.
dep_ensure/doctor/nous_subscription validate too; doctor warns on a broken link.
Closes#48521.
3c75e11571