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.

docs/CLI.md

@@ -108,7 +108,7 @@ The CLI talks to the local dashboard server — **API-backed commands require it
 | `agenthub server install-system [--dry-run\|--uninstall]` | **One Linux user per account** (S3). Run as the account that runs the dashboard, never as root: it runs `scripts/server/install-system.sh` through one `sudo`. That script creates the groups, `/srv/agenthub/users`, `/opt/agenthub`, the sudoers file, the member hooks and `hidepid`, and installs the system unit `agenthub.service`. The command then points the unit at this install, writes `AGENTHUB_USERS_ROOT`, and moves the dashboard from the `systemd --user` unit to the system one (panes stay). `--dry-run` prints the sudo command; `--uninstall` goes back to the user unit. See [SERVER-MODE.md](SERVER-MODE.md#one-linux-user-per-account) |
 | `agenthub server tools` | One Linux user per account: publish this install's scripts and every installed AI tool under `/opt/agenthub` (`tools/bin/<command>`), where every account's pane runs them. The server also does this at start and after each install or update. Prints each tool's shared copy, or why it could not be shared |
 | `agenthub server off` / `agenthub server status` | Back to local mode (accounts kept) / show the mode and account count |
-| `agenthub accounts list\|add <name>\|remove <name>` | The **Claude account pool**: new Claude sessions spread over these accounts (each a `claude setup-token` token, typed hidden or read from stdin). `list` shows live panes, $ and tokens in the last 5 h / 7 d, and quota. With `--tool codex` the same for the **Codex account pool**: `add <name> --tool codex --api-key` (key typed hidden or on stdin) or `--from <dir>` (a `codex login` home or its `auth.json`). See [SERVER-MODE.md](SERVER-MODE.md#claude-account-pool) |
+| `agenthub accounts list\|add <name>\|remove <name>` | The **Claude account pool**: new Claude sessions spread over these accounts (each a `claude setup-token` token, typed hidden or read from stdin). `list` shows live panes, $ and tokens in the last 5 h / 7 d, and quota. With `--tool codex` the same for the **Codex account pool**: `add <name> --tool codex --api-key` (key typed hidden or on stdin) or `--from <dir>` (a `codex login` home or its `auth.json`). `pin <account> <user>` / `unpin <user>` (add `--tool codex` for Codex) makes one user's new sessions always run on that account (server mode; `list` shows a `pinned` column). See [SERVER-MODE.md](SERVER-MODE.md#claude-account-pool) |
 | `agenthub users list\|add <name> [--role admin\|analyst\|member]\|passwd <name>\|role <name> <role>\|disable <name>\|enable <name>\|provision <name>\|--all\|assign <workspace id> <name>\|audit [--limit N]` | Server mode's accounts, written straight to the database. Once `install-system` ran, `add` first creates the Linux user `ah-<name>`; a name must then be lower case: a letter, then up to 28 letters, digits, `_` or `-`. `disable`/`enable` lock or unlock that Linux user too, and `provision` gives users made earlier their Linux user (idempotent); `assign` gives a workspace made before server mode an owner; `audit` lists admin/analyst reads of members' sessions. Inside a dashboard pane in server mode, every command acts as the workspace's owner through the pane's `AGENTHUB_PANE_TOKEN`. A password is typed hidden twice, or read from the first line of stdin |
 
 ### Interactive REPL