Update default anthropic model to sonnet 4.5

[krissetto]

And allow users to define their own default model in their global config

Both full model def and shorthand formats are supported

Examples of global config.yaml

version: v1
default_model:
    provider: anthropic
    model: claude-haiku-4-5
    max_tokens: 64000
    thinking_budget: 10000
    provider_opts:
        interleaved_thinking: true

version: v1
default_model: anthropic/claude-opus-4-5

[derekmisler]

/review

[github-actions]

Review Summary

Found 2 issues in the new code that should be addressed:

1. Silent failure when parsing invalid CAGENT_DEFAULT_MODEL - Users setting an invalid environment variable format won't receive any feedback
2. Potential invalid YAML output - Empty model configs could produce invalid shorthand format

The core functionality looks good, but these edge cases should be handled more gracefully.

[github-actions]

Silent failure when parsing invalid environment variable

When CAGENT_DEFAULT_MODEL is set to an invalid format (e.g., "anthropic" without /model, or "/model" without provider), parseModelShorthand returns nil silently with no warning or error. This means:

1. The user's explicit configuration is ignored without any feedback
2. The system falls back to auto-detection or user config without the user knowing
3. Users may waste time debugging why their environment variable isn't working

Recommendation: Add a warning log when the environment variable is set but parsing fails:

if model := os.Getenv(envDefaultModel); model != "" {
    runConfig.DefaultModel = parseModelShorthand(model)
    if runConfig.DefaultModel == nil {
        slog.Warn("Invalid CAGENT_DEFAULT_MODEL format, expected 'provider/model'", "value", model)
    }
} else if userCfg.DefaultModel != nil {
    runConfig.DefaultModel = &userCfg.DefaultModel.ModelConfig
}

This gives users immediate feedback when they've misconfigured the environment variable.

[github-actions]

MarshalYAML can output invalid shorthand format

isShorthandOnly() only checks if advanced fields are unset, but doesn't verify that Provider and Model are non-empty. If a FlexibleModelConfig is created with zero values (empty Provider and Model), isShorthandOnly() would return true, causing MarshalYAML to output "/" (empty + "/" + empty), which is invalid.

Recommendation: Add validation in isShorthandOnly() to ensure both fields are non-empty:

func (f *FlexibleModelConfig) isShorthandOnly() bool {
    return f.Provider != "" &&
        f.Model != "" &&
        f.Temperature == nil &&
        f.MaxTokens == nil &&
        // ... rest of checks
}

This ensures the shorthand format is only used when a valid provider/model combination exists.