feat(artifacts): storefront layout and Uninstall for installed skills/plugins

- Artifacts page laid out like an app store: hero search, category chips,
  shelves by kind, app-icon cards with one primary pill and a detail sheet
- POST /api/artifacts/:id/uninstall: a skill's copies in ~/.claude/skills and
  ~/.agents/skills are moved to ~/.claude/agenthub-replaced-skills/
  (-uninstalled-<time>), never deleted; a plugin runs
  `claude plugin uninstall --scope user --keep-data` as the caller;
  ENOTINSTALLED when nothing is there; allowed while installs are off
- Uninstall on the card, the ⋯ menu and the detail sheet, after a confirm

server/lib/artifacts.js

@@ -33,8 +33,10 @@
  *   `~/.claude/agenthub-replaced-skills/`, never deleted. A plugin is
  *   installed without `-y`, so one whose marketplace declares an install
  *   command fails rather than runs unseen. An admin can turn installs off
- *   for everyone (`setInstallsEnabled`). Share, unshare and install are
- *   written to `audit_log`.
+ *   for everyone (`setInstallsEnabled`). Uninstall (`uninstall`) takes it out
+ *   of the same accounts again — a skill moved aside the same way, a plugin
+ *   `claude plugin uninstall --keep-data` — and is allowed with installs off.
+ *   Share, unshare, install and uninstall are written to `audit_log`.
  * - Editing a skill's name, description or instructions (`editSkill`) writes
  *   a NEW snapshot and swaps the row to it; the old snapshot is dropped, and
  *   copies already installed stay as they were (Reinstall picks it up).
@@ -1409,6 +1411,31 @@ function installedCheck(req) {
   };
 }
 
+/**
+ * Move installed skill folders (`{acct, claudeHome, target}`) aside to
+ * `~/.claude/agenthub-replaced-skills/<name>[-agents][-uninstalled]-<stamp>`,
+ * as their account — a replace or an uninstall never deletes a user's copy.
+ */
+async function moveAside(taken, row, label = "") {
+  const runAs = require("./run-as");
+  const stamp = new Date().toISOString().replace(/[:.]/g, "-");
+  for (const t of taken) {
+    const tag = t.target.startsWith(t.claudeHome + path.sep) ? "" : "-agents";
+    const aside = path.join(
+      t.claudeHome,
+      "agenthub-replaced-skills",
+      `${row.source}${tag}${label}-${stamp}`
+    );
+    if (t.acct) {
+      runAs.mkdirAs(t.acct, path.dirname(aside));
+      await runAs.writeAsAsync(t.acct, t.target, ["move", aside]);
+    } else {
+      fs.mkdirSync(path.dirname(aside), { recursive: true });
+      fs.renameSync(t.target, aside);
+    }
+  }
+}
+
 /**
  * Install a skill snapshot as `<name>` into every skills folder of every
  * account the viewer's sessions run as (`installAccounts` × `skillDirsOf`).
@@ -1428,22 +1455,7 @@ async function installSkill(req, row, { replace = false } = {}) {
       "replacing needs the updated helper: run `agenthub server install-system` once"
     );
   }
-  const stamp = new Date().toISOString().replace(/[:.]/g, "-");
-  for (const t of taken) {
-    const tag = t.target.startsWith(t.claudeHome + path.sep) ? "" : "-agents";
-    const aside = path.join(
-      t.claudeHome,
-      "agenthub-replaced-skills",
-      `${row.source}${tag}-${stamp}`
-    );
-    if (t.acct) {
-      runAs.mkdirAs(t.acct, path.dirname(aside));
-      await runAs.writeAsAsync(t.acct, t.target, ["move", aside]);
-    } else {
-      fs.mkdirSync(path.dirname(aside), { recursive: true });
-      fs.renameSync(t.target, aside);
-    }
-  }
+  await moveAside(taken, row);
   const files = row.files ? JSON.parse(row.files) : [];
   const contents = files.map((f) => ({
     rel: f.path.split("/"),
@@ -1470,14 +1482,9 @@ function __setExecImpl(fn) {
   execImpl = fn;
 }
 
-/** `claude plugin marketplace add` + `claude plugin install --scope user`, as the viewer. */
-async function installPlugin(req, row) {
+/** `args => Promise`: runs `claude <args>` as the viewer's own account. */
+function claudeAsViewer(req) {
   const { acct } = ownAccount(req);
-  const steps = [
-    ["plugin", "marketplace", "add", validMarket(row.market)],
-    ["plugin", "install", "--scope", "user", row.source],
-  ];
-  if (!PLUGIN_RE.test(row.source)) throw fail("EBADPLUGIN", "plugin: name@marketplace");
   let run;
   if (acct) {
     const runAs = require("./run-as");
@@ -1493,11 +1500,23 @@ async function installPlugin(req, row) {
     if (process.env.CLAUDE_HOME) env.CLAUDE_CONFIG_DIR = getClaudeHome();
     run = (args) => execImpl("claude", args, { timeout: 300_000, encoding: "utf8", env });
   }
-  const lastLine = (e) =>
-    String(e?.stderr || e?.stdout || e?.message || "")
-      .trim()
-      .split("\n")
-      .pop();
+  return run;
+}
+
+const lastLine = (e) =>
+  String(e?.stderr || e?.stdout || e?.message || "")
+    .trim()
+    .split("\n")
+    .pop();
+
+/** `claude plugin marketplace add` + `claude plugin install --scope user`, as the viewer. */
+async function installPlugin(req, row) {
+  const steps = [
+    ["plugin", "marketplace", "add", validMarket(row.market)],
+    ["plugin", "install", "--scope", "user", row.source],
+  ];
+  if (!PLUGIN_RE.test(row.source)) throw fail("EBADPLUGIN", "plugin: name@marketplace");
+  const run = claudeAsViewer(req);
   try {
     await run(steps[0]);
   } catch (e) {
@@ -1527,6 +1546,58 @@ async function install(req, id, opts = {}) {
   return result;
 }
 
+/**
+ * Take an installed skill or plugin out of the viewer's own accounts again
+ * (`installAccounts`). A skill's folders are moved aside like a replace
+ * (`moveAside`, `-uninstalled`), never deleted; a plugin is `claude plugin
+ * uninstall --scope user --keep-data`, as the viewer, so its data dir stays.
+ * Allowed while installs are off — it only removes. `ENOTINSTALLED` when
+ * there is nothing of it to remove.
+ */
+async function uninstall(req, id) {
+  const row = load(req, id);
+  if (row.kind !== "skill" && row.kind !== "plugin") {
+    throw fail("EBADKIND", "only a skill or a plugin is uninstalled");
+  }
+  let result;
+  if (row.kind === "skill") {
+    const runAs = require("./run-as");
+    const taken = installAccounts(req)
+      .flatMap((a) => skillDirsOf(a).map((dir) => ({ ...a, target: path.join(dir, row.source) })))
+      .filter((t) => exists(t.target));
+    if (!taken.length) throw fail("ENOTINSTALLED", `${row.source} is not installed`);
+    if (taken.some((t) => t.acct) && !runAs.writeAsCanMove()) {
+      throw fail(
+        "EUPGRADE",
+        "uninstalling needs the updated helper: run `agenthub server install-system` once"
+      );
+    }
+    await moveAside(taken, row, "-uninstalled");
+    result = { uninstalled: row.source, paths: taken.map((t) => t.target) };
+  } else {
+    if (!PLUGIN_RE.test(row.source)) throw fail("EBADPLUGIN", "plugin: name@marketplace");
+    try {
+      await claudeAsViewer(req)([
+        "plugin",
+        "uninstall",
+        "--scope",
+        "user",
+        "--keep-data",
+        row.source,
+      ]);
+    } catch (e) {
+      const msg = lastLine(e);
+      if (/not (found|installed)/i.test(msg)) {
+        throw fail("ENOTINSTALLED", `${row.source} is not installed`);
+      }
+      throw fail("EPLUGIN", `uninstall: ${msg}`);
+    }
+    result = { uninstalled: row.source, paths: [] };
+  }
+  if (req?.user && row.owner_id !== req.user.id) audit(req, row, "uninstalled");
+  return result;
+}
+
 /**
  * Who installed artifact `id`, newest first: `{items: [{user, mine, at,
  * replaced, paths?}], unrecorded}`. The owner, staff and local mode see every
@@ -1593,6 +1664,7 @@ module.exports = {
   remove,
   snapshotFile,
   install,
+  uninstall,
   installsEnabled,
   setInstallsEnabled,
   __setExecImpl,