chore(gitignore): ignore installer .install_method stamp (salvage of #54855) (#67364)

* chore(gitignore): ignore installer .install_method stamp

Salvage of #54855 by @drissman — rebased onto current main with root-scoped
rule and sister-marker comments alongside .update-incomplete.

Closes #66189

Root cause: scripts/install.sh writes <install>/.install_method but git
did not ignore it, so managed checkouts show ?? .install_method and
hermes update may autostash the untracked marker.

Fix: add /.install_method to .gitignore (repo-root only).

Verification: git check-ignore -v .install_method

* test(update): assert .install_method survives update autostash (#66189)

Add hermetic regression mirroring the .hermes-bootstrap-complete test:
adopt the real .gitignore, drop the installer .install_method stamp, run
the exact 'git stash push --include-untracked' the updater uses, and assert
the marker is neither swept nor reported dirty. Requested by hermes-sweeper
review on #67364.

.gitignore

@@ -150,6 +150,11 @@ docs/superpowers/*
 .update-incomplete
 .update-incomplete.lock
 
+# Installer-written method stamp in the managed checkout root (scripts/install.sh).
+# Runtime metadata only — never a code change. Ignore so `git status` stays clean
+# and `hermes update`'s untracked autostash does not treat it as a local edit (#66189 / #54855).
+/.install_method
+
 # Tool Search live-test harness output — non-deterministic model transcripts,
 # regenerated by scripts/tool_search_livetest.py. Never an artifact of the repo.
 scripts/out/