Agent config.yaml: parsing and model resolution

[ludusrusso]

Parent PRD

#34

What to build

A config type for parsing a static agent's config.yaml file and resolving its model reference against the global configuration. Minimal for v1: only a model field.

model: gemini/gemini-2.0-flash

When model is omitted, the agent inherits the global default_model.

Acceptance criteria

• New config type (e.g., in pkg/agent/) that parses agent config.yaml
• model field parsed as a string (provider/model reference or alias)
• When model is empty or file is missing, falls back to global default_model
• Config file is loaded from a Home directory (uses Home.Dir() path)
• Tests: parse valid config, missing model defaults, missing file defaults, invalid YAML errors

Blocked by

None — can start immediately.

User stories addressed

• User story 6 (per-agent model config)