Fix gateway advertising 'unknown' version in production installs

hermes plugins install <repo>#gateway-plugin ships ONLY the
gateway-plugin/ subdirectory into ~/.hermes/plugins/iris, so the
repo-root VERSION file is not present there and plugin_version()
fell back to 'unknown' — the app then showed a false
'App and gateway versions differ' warning.

- version.py: resolution chain repo-root VERSION (dev/symlink
  install) -> plugin.yaml version field (production install) ->
  'unknown'; stdlib-only parse, base= param for tests
- plugin.yaml: bump stale version 0.1.0 -> 0.1.2 (matches VERSION)
- scripts/check_version_sync.sh + pre-commit hook: fail commits
  where plugin.yaml drifts from the repo-root VERSION
- tests: regression test simulating the production layout

.pre-commit-config.yaml

@@ -12,4 +12,10 @@ repos:
         entry: scripts/guard_hermes_agent.sh --staged
         language: system
         pass_filenames: false
-        always_run: true
+        always_run: true
+      - id: check-version-sync
+        name: check gateway-plugin/plugin.yaml version == repo-root VERSION
+        entry: scripts/check_version_sync.sh
+        language: system
+        pass_filenames: false
+        always_run: true