fix: plugin-install modules no longer start the update journeys they never reach

Replaying the last 131 non-merge main commits (4787e4d56f) through the
classifier: main selects e2e_upgrade 5 / e2e_desktop_update 3, the CI branch
12 / 10. Four of the seven new hits per lane touched only plugins_cmd.py or
plugins_cmd_install.py. migrate_all_homes reaches them only when a home's
configured memory provider left core (the catalog install); no upgrade or
Desktop update journey configures one, so those suites cannot observe the
change, and the unit lane already runs on every .py edit. Measured on a
fresh home: memory_provider_migration, pm/plugins_state, agent/memory_provider
and plugins/memory run; plugins_cmd_install executes only its module body,
plugins_cmd is never imported.

Kept, because a fresh-home update runs them: memory_provider_migration.py
(migrate_all_homes on every source build), plugins/memory/__init__.py (the
post-update Honcho profile sync), pm/install.py (sealed() /
lazy_installs_allowed() gate every update's default-tool install), and
scripts/build/ (frontend-common.mjs is imported by freshness/web/tui/desktop).
After: 8 / 6. A guard test fails if the journey-home migration starts running
plugin-install code, the signal to route those modules again.

scripts/ci/classify_changes.py

@@ -290,12 +290,13 @@ _UPDATE_DEPENDENCIES = (
     "hermes_cli/_subprocess_compat.py",  # update git env, process-tree kill, PM git exposure
     "hermes_cli/local_runtime/processes.py",  # bounded probes' spawn_server/job custody
     "agent/deadline.py",  # bounded probes' process-tree timeout cleanup
-    # migrate_all_homes' second-hop provider/profile/install decisions.
+    # migrate_all_homes' second-hop provider/profile decisions, run on every update. Its
+    # plugin-install branch (plugins_cmd, plugins_cmd_install) runs only for a home whose
+    # configured memory provider left core; no update journey's home has one, so those
+    # modules stay with the unit lane (tests/ci/test_update_transitive_routing.py).
     "agent/memory_provider.py",
-    "hermes_cli/plugins_cmd_install.py",
-    "hermes_cli/plugins_cmd.py",
     "pm/plugins_state.py",
-    "pm/install.py",
+    "pm/install.py",  # also sealed()/lazy_installs_allowed(): every update's default-tool install
     "hermes_cli/desktop_build_lock.py",
     "hermes_cli/memory_provider_migration.py",
     "hermes_cli/left_core_migration.py",  # source_build migrates plugins that left core