`hermes dashboard` is a long-lived foreground server that users often
start and forget about, sometimes in a shell they've since closed. We
didn't have a way to stop it — users had to find the PID manually.
Adds two lifecycle flags that reuse the same detection + termination
path the post-`hermes update` cleanup (PR #17832) uses:
hermes dashboard --status
List running hermes dashboard processes with PID + cmdline.
Exit 0, informational.
hermes dashboard --stop
Terminate all running dashboards (3s grace then force-kill survivors).
Exit 0 if none remain, 1 if any couldn't be stopped.
Windows uses `taskkill /F` as before.
Both flags short-circuit before any fastapi/uvicorn import so they work
even on installations where the dashboard extras aren't installed —
useful when you're cleaning up after uninstalling.
The kill helper gained an optional `reason=...` param so the output
reads "(requested via --stop)" instead of the post-update-specific
"running backend no longer matches the updated frontend" wording.
E2E: `hermes dashboard --status` with nothing running prints the
empty message; with a fake `hermes dashboard ...` cmdline spawned via
`exec -a`, `--status` lists it, `--stop` terminates it (exit -15),
and a follow-up `--status` returns empty.
