Implement config layering and override resolution

[wauputr4]

Goal

Make configuration adjustable without hardcoding environment-specific values.

Scope

• Resolve config in this order: profile defaults, .wasila/config.toml, environment variables, CLI flags.
• Keep provider secrets in env vars only.
• Support OpenAI-compatible provider fields: base_url, model, api_key_env.
• Keep customer gateway and owner gateway config independent.

References

• docs/11-configuration.md
• docs/15-provider-configuration.md
• docs/22-architecture-boundaries.md

Acceptance Criteria

• CLI flags can override local config where supported.
• Env vars supply secret values without writing them into config.
• Provider config is swappable without changing profile files.
• Customer and owner gateway config remain independent.

[wauputr4]

Provider handling has been aligned with CrewAI in ce88551: Wasila provider config now translates to CrewAI LLM kwargs, default model uses CrewAI-style provider prefix (openai/gpt-4.1-mini), and docs clarify that Wasila should use CrewAI LLM rather than a custom provider HTTP client.