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

gateway-plugin/plugin.yaml

@@ -1,7 +1,11 @@
 name: iris-platform
 label: Iris
 kind: platform
-version: 0.1.0
+# MUST match the repo-root VERSION file (checked by
+# scripts/check_version_sync.sh on commit). This field is the version the
+# gateway advertises in production installs, where only this plugin dir is
+# shipped (see version.py).
+version: 0.1.2
 description: >
   Native Android / Desktop client gateway adapter for Hermes Agent.
   Runs an HTTP server (optional TLS) inside the gateway; the app connects