Adds systemd --user bus-connection troubleshooting

Enabling lingering doesn't start the user manager retroactively, and
sudo -u often drops XDG_RUNTIME_DIR - both produce the same "Failed
to connect to bus" error when setting this up by hand.

aidlc-docs/features/gitea-deployment-workflow/operations/deployment/deployment-instructions.md

@@ -65,6 +65,19 @@ common "Failed to connect to bus" error `systemctl --user` can throw when invoke
 non-interactive SSH command — lingering keeps the user's systemd instance (and `XDG_RUNTIME_DIR`)
 running independent of any login session.
 
+**If `systemctl --user ...` still fails with `Failed to connect to bus: No medium found`** while
+setting the rest of this up manually (before the deploy workflow ever runs): enabling lingering
+does not retroactively start the user manager — that happens on the next real login, reboot, or
+manually:
+```bash
+sudo systemctl start user@$(id -u gitea-workflow).service
+```
+And run `systemctl --user` commands from a **real login shell** for that account
+(`sudo -i -u gitea-workflow`), not `sudo -u gitea-workflow systemctl --user ...` from your own
+session — the latter often doesn't carry `XDG_RUNTIME_DIR` along, which produces this exact error.
+Verify with `loginctl show-user gitea-workflow | grep Linger` (expect `Linger=yes`) and
+`ls /run/user/<uid>` (should exist once the user manager has actually started).
+
 ### 1.4 Directory Skeleton
 
 Deliberately placed under `gitea-workflow`'s **own home directory**, not under