Markdown Edit/Preview tabs + read-only .md viewer for skills & profiles

Adds MarkdownEditor (react-markdown + remark-gfm, no raw HTML — authored/retrieved
content is data, not markup) with Edit | Preview tabs, wired into the AGENTS.md and
SKILL.md editors, the agent persona, and the review artifact.

Adds a read-only "View" on every skill and agent-profile card — including builtins,
which previously had no way to be inspected at all — rendering the full SKILL.md /
AGENTS.md (frontmatter + body + actions/golden tests). Collapses a same-version
builtin that an org has forked so its own copy shadows it, keeping the version
picker unambiguous and the item clearly editable/versionable.

Also lands the agent-face wiring on the seat configurator (a live xl preview with a
state cycler) and the review inbox header.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/package.json

@@ -24,8 +24,10 @@
     "react": "^19.2.6",
     "react-dom": "^19.2.6",
     "react-hook-form": "^7.78.0",
+    "react-markdown": "^10.1.0",
     "react-router": "^7.17.0",
     "recharts": "^3.8.1",
+    "remark-gfm": "^4.0.1",
     "shadcn": "^4.11.0",
     "sonner": "^2.0.7",
     "tailwind-merge": "^3.6.0",