PR #30136 review caught: `S6ServiceManager.start/stop/restart` called
`subprocess.run(check=True)` on `s6-svc`, so any failure surfaced as
a raw `CalledProcessError` traceback. The two cases operators
actually hit are:
1. The service slot doesn't exist — most commonly because the user
typed a profile name wrong (`hermes -p typo gateway start`).
2. s6-svc itself fails — most commonly EACCES on the supervise
control FIFO when running unprivileged.
Both deserve named errors with actionable messages, not stacktraces.
Changes:
* Add `S6Error` base + two concrete errors in `hermes_cli.service_manager`:
- `GatewayNotRegisteredError(profile)` — carries the unprefixed
profile name; message: `no such gateway 'typo': register it
with `hermes profile create typo` first, or pass an existing
profile name via `-p <name>``.
- `S6CommandError(service, action, returncode, stderr)` — carries
the s6-svc rc and stderr; message: `s6-svc start on
'gateway-coder' failed (rc=111): <stderr>`.
* Factor lifecycle dispatch through `_run_svc(flag, label, name)`:
pre-checks that the service directory exists (raises
GatewayNotRegisteredError before invoking s6-svc), then runs
s6-svc and translates any CalledProcessError into S6CommandError.
* `_dispatch_via_service_manager_if_s6` in `hermes_cli.gateway`
catches both errors and prints `✗ <message>` + `sys.exit(1)`
instead of letting the exception bubble. The dispatch path that
used to dump a traceback at the user now gives an actionable
one-liner.
Tests: 6 new tests for the error types and their CLI rendering;
existing lifecycle test pre-seeds the slot directory before calling
`mgr.start` etc.
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