feat(doctor): report the installed plugin generation, not just its presence

Every other check in doctor is about symlink paths. That is the wrong shape
for Claude Code, which has no symlinks — it has the plugin, and it gates
updates on plugin.json's version. A host that never picked up a release loads
every skill successfully and is simply a generation behind, silently.

Found live while writing this: koala had skills@mathias 0.6.0 installed, a
marketplace clone at 0.7.0 and a checkout at 0.8.4, and doctor said "clean".

doctor now compares the version recorded in installed_plugins.json against the
newest manifest already on the host — the active checkout and any marketplace
clone, filtered to plugin.json files that actually name this plugin — and fails
naming both versions. Offline, like the rest of doctor: "available" never means
a fetch.

A host with no install record is a valid symlink-only configuration and stays
clean. --json grows a "plugin":{"installed","available"} object so per-host
results can be collected without parsing prose.

Four new fixture cases: match, stale, absent record, newer marketplace clone.
Live output on koala:

  FAIL plugin-version: Claude Code has skills@mathias 0.6.0 installed but
  0.8.4 is available on this host

Closes #30
Bump-Type: minor

bin/doctor

@@ -180,10 +180,63 @@ if [ -f "$INSTALLED" ]; then
     || add WARN plugin "skills@mathias is not installed for Claude Code — /plugin marketplace add https://git.d-ma.be/mathias/skills.git"
 fi
 
+# --- Claude Code: which GENERATION of the plugin is installed ---------------
+# Every check above is about symlink paths, which is the wrong shape for the
+# plugin: it is the only channel Claude Code has, and Claude Code gates updates
+# on plugin.json's version. A host that never picked up a release loads all the
+# skills successfully and is simply a generation behind — silently. Found on
+# koala 2026-08-29: installed 0.6.0, marketplace clone 0.7.0, repo 0.8.5,
+# doctor said "clean" (#30).
+#
+# Offline by construction, like the rest of doctor: "available" means the
+# newest manifest already on this host — the active checkout and any
+# marketplace clone — never a fetch.
+manifest_version() {
+  [ -f "$1" ] || return 1
+  grep -q '"name"[[:space:]]*:[[:space:]]*"skills"' "$1" || return 1
+  sed -n 's/.*"version"[[:space:]]*:[[:space:]]*"\([^"]*\)".*/\1/p' "$1" | head -1
+}
+
+# newer_version a b — echoes whichever sorts higher on major.minor.patch.
+newer_version() {
+  printf '%s\n%s\n' "$1" "$2" | awk -F. '
+    { raw[NR] = $0; for (i = 1; i <= 3; i++) n[NR, i] = ($i == "" ? 0 : $i + 0) }
+    END {
+      for (i = 1; i <= 3; i++) {
+        if (n[1, i] > n[2, i]) { print raw[1]; exit }
+        if (n[1, i] < n[2, i]) { print raw[2]; exit }
+      }
+      print raw[1]
+    }'
+}
+
+installed_v=''
+if [ -f "$INSTALLED" ]; then
+  installed_v=$(sed -n '/"skills@mathias"/,/\]/p' "$INSTALLED" \
+    | sed -n 's/.*"version"[[:space:]]*:[[:space:]]*"\([^"]*\)".*/\1/p' | head -1)
+fi
+
+available_v=''
+for manifest in "$CHECKOUT/.claude-plugin/plugin.json" \
+                "$HOME"/.claude/plugins/marketplaces/*/.claude-plugin/plugin.json
+do
+  v=$(manifest_version "$manifest") || continue
+  [ -n "$v" ] || continue
+  if [ -z "$available_v" ]; then
+    available_v=$v
+  else
+    available_v=$(newer_version "$available_v" "$v")
+  fi
+done
+
+if [ -n "$installed_v" ] && [ -n "$available_v" ] && [ "$installed_v" != "$available_v" ]; then
+  add FAIL plugin-version "Claude Code has skills@mathias $installed_v installed but $available_v is available on this host — /plugin update skills@mathias (a host stale enough to matter is running the old doctor, so the first detection may need this run by hand)"
+fi
+
 # --- output -----------------------------------------------------------------
 if [ "$JSON" -eq 1 ]; then
-  printf '{"checkout":"%s","expected":%s,"findings":[%s]}\n' \
-    "$CHECKOUT" "$EXPECTED" "${report%,}"
+  printf '{"checkout":"%s","expected":%s,"plugin":{"installed":"%s","available":"%s"},"findings":[%s]}\n' \
+    "$CHECKOUT" "$EXPECTED" "$installed_v" "$available_v" "${report%,}"
 elif [ "$findings" -gt 0 ]; then
   printf 'skills doctor — %s finding(s), checkout %s\n\n%s\n' \
     "$findings" "$CHECKOUT" "$report"