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

scripts/check_version_sync.sh

@@ -0,0 +1,41 @@
+#!/usr/bin/env bash
+# Fail the commit if gateway-plugin/plugin.yaml's `version:` field drifts
+# from the repo-root VERSION file (the single source of truth).
+#
+# Why: `hermes plugins install <repo>#gateway-plugin` ships ONLY the
+# gateway-plugin/ subdirectory into ~/.hermes/plugins/iris, so in production
+# the gateway advertises the version from plugin.yaml (see
+# gateway-plugin/version.py). If the two drift, the app shows a false
+# "versions differ" warning.
+set -euo pipefail
+
+repo_root="$(git rev-parse --show-toplevel)"
+version_file="$repo_root/VERSION"
+plugin_yaml="$repo_root/gateway-plugin/plugin.yaml"
+
+[ -f "$version_file" ] || {
+    echo "check_version_sync: missing $version_file" >&2
+    exit 1
+}
+[ -f "$plugin_yaml" ] || {
+    echo "check_version_sync: missing $plugin_yaml" >&2
+    exit 1
+}
+
+root_version="$(tr -d '[:space:]' <"$version_file")"
+# Mirrors gateway-plugin/version.py's _VERSION_RE: optional single OR double
+# quote, at least one captured character.
+yaml_version="$(sed -n "s/^version:[[:space:]]*[\"']\{0,1\}\([^\"'[:space:]]\{1,\}\).*/\1/p" "$plugin_yaml" | head -n1)"
+
+if [ -z "$yaml_version" ]; then
+    echo "check_version_sync: no top-level 'version:' field in gateway-plugin/plugin.yaml" >&2
+    exit 1
+fi
+
+if [ "$root_version" != "$yaml_version" ]; then
+    echo "check_version_sync: version drift" >&2
+    echo "  VERSION (repo root)          = $root_version" >&2
+    echo "  gateway-plugin/plugin.yaml   = $yaml_version" >&2
+    echo "Bump both to the same value (VERSION is the source of truth)." >&2
+    exit 1
+fi