Phase 4 of the /model command overhaul.
Both the CLI (cli.py) and gateway (gateway/run.py) /model handlers
had ~50 lines of duplicated core logic: parsing, provider detection,
credential resolution, and model validation. This extracts that
pipeline into hermes_cli/model_switch.py.
New module exports:
- ModelSwitchResult: dataclass with all fields both handlers need
- CustomAutoResult: dataclass for bare '/model custom' results
- switch_model(): core pipeline — parse → detect → resolve → validate
- switch_to_custom_provider(): resolve endpoint + auto-detect model
The shared functions are pure (no I/O side effects). Each caller
handles its own platform-specific concerns:
- CLI: sets self.model/provider/etc, calls save_config_value(), prints
- Gateway: writes config.yaml directly, sets env vars, returns markdown
Net result: -244 lines from handlers, +234 lines in shared module.
The handlers are now ~80 lines each (down from ~150+) and can't drift
apart on core logic.
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