feat(discord): systemd user service with a supervised run; brakes exit 3 and are never retried (#1509)

QUEUE row 17, MVP iteration 2. scripts/discord-service.sh renders and
installs mosaic-discord@<binding> from packages/discord/systemd/. The
unit's main process is `run --supervised`, which applies the new recover
policy first: a lock whose owner is gone is cleared and only the STOP
written for that is removed; an operator STOP or a held binding refuses
with exit 3, which RestartPreventExitStatus never retries. `recover` is
also a CLI verb. First cut used ExecStartPre and looped live, since systemd
honours the never-retry status only from the main process; replaced and
re-verified before any message traffic. Suite 40/40, 95 node tests.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

packages/discord/src/errors.mjs

@@ -1,5 +1,6 @@
 // One error class for the package. exitCode follows docs/TOOLS.md: 1 operation
-// failed, 2 invalid data or configuration, 4 usage.
+// failed, 2 invalid data or configuration, 3 refused by a brake (STOP or a
+// held binding; a supervisor must not retry), 4 usage.
 export class DiscordError extends Error {
   constructor(message, exitCode = 2, details = undefined) {
     super(message);