Support model

[nathro]

Add model selection support to the GitHub Action and Auggie CLI invocation

• Introduce optional model input in the action (action.yml) and expose it via INPUT_MODEL
• Parse and validate model across the codebase: ActionInputs, Zod ActionInputsSchema, and INPUT_FIELD_MAP
• Pass --model <value> to auggie when the input is provided, for both instruction-file and direct-instruction modes
• Document the new model input in the README inputs table with an example default (gpt-4o-mini)
• Maintain backward compatibility: existing workflows work unchanged when model is omitted

Business value

• Enable workflows to choose the LLM/model per run to balance cost vs. quality and align with org defaults

Breaking changes

• None

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