Makes hermes -z usable by sweeper without mutating user config.
- Top-level -m/--model and --provider flags that apply to -z/--oneshot
(mirrors hermes chat's plumbing).
- HERMES_INFERENCE_MODEL env var as the parallel to HERMES_INFERENCE_PROVIDER
for CI / scripted invocations.
- resolve_runtime_provider() gets the requested provider; when --model is
given without --provider, detect_provider_for_model() auto-selects the
provider that serves it (same semantic as /model in an interactive session).
- --provider without --model errors out with exit 2 — carrying a config
model across to a different provider is usually wrong, and silently
picking the provider's catalog default hides the mismatch.
Config defaults still used when both flags are omitted (existing behavior).
Validation (all live against OpenRouter):
-z 'x' ....................... uses config default (opus-4.7)
-z 'x' --model haiku-4.5 ..... haiku-4.5 via auto-detected openrouter
-z 'x' --model ... --provider pair as given
HERMES_INFERENCE_MODEL=... -z haiku-4.5 via env var
-z 'x' --provider anthropic .. exits 2 with error to stderr
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