feat(plh): flag plugin namespace collisions (same declared name)

Two plugins that declare the same `name` in plugin.json collapse into one
component namespace (/name:command, name:skill, agent "name"). Resolution
between two installed same-name plugins is undocumented, so one plugin's
commands/skills/agents are silently shadowed and unreachable. PLH now flags
this at medium severity, keying on the declared `name` (not folder basename,
via new declaredName on scanSinglePlugin) with a COL-shaped details.namespaces
payload. Name-less plugins are excluded from the collision map.

Search-first (code.claude.com/docs/en/plugins): plugin components are
namespaced by the declared name, so a plugin component can never shadow a
user/project one — only a same-name collision loses components. This refutes
the original "plugin vs user vs project shadowing" framing in the backlog.

Adds humanizer pattern, fixture (duplicate-plugin-name: 2 colliding + 2
name-less), and 3 tests. Suite 929->932. self-audit A 97 / A 100, scanners 13.

scanners/lib/humanizer-data.mjs

@@ -723,6 +723,14 @@ export const TRANSLATIONS = {
           recommendation: 'Add the missing setting shown in the details.',
         },
       },
+      {
+        regex: /^Plugin namespace collision:/,
+        translation: {
+          title: 'Two plugins share the same namespace, so one hides the other',
+          description: 'Claude Code names a plugin\'s commands, skills, and agents after the plugin (like `/name:command`). When two plugins declare the same name, they share one namespace and only one is reachable — the other\'s commands, skills, and agents silently disappear.',
+          recommendation: 'Give each plugin a distinct `name` in its `plugin.json`. The folder name doesn\'t matter — the `name` field is what forms the namespace.',
+        },
+      },
     ],
     _default: {
       title: 'A plugin has a configuration issue',