feat(pool): pin a user to one Claude/Codex pool account

An admin pins a server-mode user to one pool account per tool
(account_pins; PUT /api/accounts/pins with the password step-up and audit,
the Users page's Pin a user, agenthub accounts pin|unpin). The workspace
owner's pin wins over the balancer whenever the pane may use the account;
a pinned login still waits while another pool login holds the home.

bin/agenthub.js

@@ -2855,8 +2855,8 @@ const COMMAND_GROUPS = [
       ],
       [
         "accounts",
-        "list|add|remove [name] [--tool codex]",
-        "Claude/Codex account pool: new sessions spread over these accounts",
+        "list|add|remove [name]|pin <account> <user>|unpin <user> [--tool codex]",
+        "Claude/Codex account pool: new sessions spread over these accounts; pin = one user always on one",
       ],
       ["repl", "", "Open the interactive shell (also: shell, i)"],
     ],
@@ -3814,7 +3814,7 @@ const SUBCOMMANDS = {
   service: ["install", "uninstall", "status"],
   server: ["setup", "off", "status"],
   users: ["list", "add", "passwd", "role", "disable", "enable", "assign", "audit"],
-  accounts: ["list", "add", "remove"],
+  accounts: ["list", "add", "remove", "pin", "unpin"],
   pricing: ["set", "delete", "reset"],
   webhooks: ["test"],
   import: ["rescan", "path"],
@@ -4286,7 +4286,18 @@ async function cmdCodexAccounts(sub, name, flags) {
   }
   pool.resetQuota();
   table(
-    ["codex account", "kind", "live", "$ 5h", "$ 7d", "tok 5h", "tok 7d", "quota", "state"],
+    [
+      "codex account",
+      "kind",
+      "live",
+      "$ 5h",
+      "$ 7d",
+      "tok 5h",
+      "tok 7d",
+      "quota",
+      "state",
+      "pinned",
+    ],
     pool
       .poolStatus()
       .map((a) => [
@@ -4301,10 +4312,53 @@ async function cmdCodexAccounts(sub, name, flags) {
           ? a.quota.windows.map((w) => `${w.label} ${Math.round(w.usedPercent)}%`).join(" ")
           : "–",
         a.full ? "full" : "ok",
+        pinnedUsers("codex", a.name),
       ])
   );
 }
 
+/**
+ * `agenthub accounts pin <account> <user>` / `unpin <user>` [--tool codex] —
+ * that user's new sessions run on that account (server/lib/account-pins.js).
+ */
+function cmdAccountPin(sub, positional, tool) {
+  const pins = require(path.join(REPO_ROOT, "server", "lib", "account-pins.js"));
+  const [first, second] = positional;
+  const username = sub === "pin" ? second : first;
+  if (!username || (sub === "pin" && !first)) {
+    throw new Error("usage: agenthub accounts pin <account> <user> | unpin <user> [--tool codex]");
+  }
+  const user = require(path.join(REPO_ROOT, "server", "db.js"))
+    .db.prepare("SELECT id FROM users WHERE username = ?")
+    .get(username);
+  if (!user) throw new Error(`no user ${username}`);
+  const pin = pins.setPin(user.id, tool, sub === "pin" ? first : null);
+  console.log(
+    pin.account
+      ? `${c.green("✔")} ${pin.username}'s new ${tool} sessions run on ${pin.account}`
+      : `${c.green("✔")} ${pin.username} unpinned — new ${tool} sessions use the balancer`
+  );
+  if (!require(path.join(REPO_ROOT, "server", "lib", "auth.js")).isServerMode()) {
+    console.log(c.dim("  Pins apply in server mode only."));
+  }
+  console.log(c.dim("  Running sessions keep their account until they end."));
+}
+
+/** The `pinned` column: who an admin pinned to `account`. */
+function pinnedUsers(tool, account) {
+  try {
+    return (
+      require(path.join(REPO_ROOT, "server", "lib", "account-pins.js"))
+        .listPins(tool)
+        .filter((p) => p.account === account)
+        .map((p) => p.username)
+        .join(",") || "–"
+    );
+  } catch {
+    return "–";
+  }
+}
+
 /** `agenthub accounts …` — the Claude account pool (server/lib/claude-pool.js). */
 async function cmdAccounts(sub, positional, flags = {}) {
   serverAuth(); // the same settings + database the server reads
@@ -4316,6 +4370,8 @@ async function cmdAccounts(sub, positional, flags = {}) {
   if (flags.tool != null && flags.tool !== "claude" && flags.tool !== "codex") {
     throw new Error("--tool is claude (the default) or codex");
   }
+  if (sub === "pin" || sub === "unpin")
+    return cmdAccountPin(sub, positional, flags.tool || "claude");
   if (flags.tool === "codex") return cmdCodexAccounts(sub, name, flags);
   if (sub === "add") {
     console.log(
@@ -4337,7 +4393,18 @@ async function cmdAccounts(sub, positional, flags = {}) {
     await pool.refreshQuotas({ force: true });
     const pct = (w) => (w?.utilization != null ? `${Math.round(w.utilization)}%` : "–");
     table(
-      ["account", "live", "$ 5h", "$ 7d", "tok 5h", "tok 7d", "quota 5h", "quota wk", "state"],
+      [
+        "account",
+        "live",
+        "$ 5h",
+        "$ 7d",
+        "tok 5h",
+        "tok 7d",
+        "quota 5h",
+        "quota wk",
+        "state",
+        "pinned",
+      ],
       pool
         .poolStatus()
         .map((a) => [
@@ -4356,6 +4423,7 @@ async function cmdAccounts(sub, positional, flags = {}) {
               : a.quota?.error
                 ? `no quota (${a.quota.error})`
                 : "ok",
+          pinnedUsers("claude", a.name),
         ])
     );
     const codex = require(path.join(REPO_ROOT, "server", "lib", "codex-pool.js"));
@@ -4363,7 +4431,9 @@ async function cmdAccounts(sub, positional, flags = {}) {
     return;
   }
   console.error(
-    c.red("✖ Usage: agenthub accounts list | add <name> | remove <name> [--tool codex]")
+    c.red(
+      "✖ Usage: agenthub accounts list | add <name> | remove <name> | pin <account> <user> | unpin <user> [--tool codex]"
+    )
   );
   process.exitCode = 1;
 }