agents: pull the forge token from bao; drop tea-login

forge-token.nix fetches swarm/agents/<agent>/forge-token under the
agent's own store identity into /run/hive-agent-forge-token/token, and
re-fetches on a timer so a rotation lands. hive-forge, the git
credential helper, hive-forge-notify, forge-avatar-sync and the web UI
read that file first and fall back to <state>/forge-token.

tea-login is deleted: it copied the token into ~/.config/tea, which
docs/swarm/credentials.md forbids for a store secret. hive-forge covers
the same verbs. swarmctl gains agent mint-forge-token.

Refs #3782

docs/process/gotchas.md

@@ -448,7 +448,7 @@ connects to the compositor at `127.0.0.1:<vnc_port>`.
   must never block on weston signalling readiness. A misconfigured
   weston degrades to a `Restart=on-failure` loop visible in
   `journalctl`, it doesn't abort the `nixos-container update`.
-  Same reasoning as the `tea-login` unit in `nix/agent-modules/forge.nix`.
+  Same reasoning as the best-effort oneshots in `docs/process/conventions.md`.
 - **`[core] idle-time=0`**: disables weston's 300-second idle
   timeout. Without it the VNC desktop fades to black and
   desktop-shell shows its click-to-unlock screen — useless for an