Document global YAML defaults vs per-agent overrides in settings docs

[rosspeili]

Problem Statement

Per-agent timeout is documented in README and docs/EXAMPLES.md, but it is not obvious that defaults.timeout, defaults.litellm_model, and defaults.temperature in rooms.settings.yaml apply to shipped personas and the orchestrator unless overridden in the wizard. New users may edit only per-agent fields in-session and miss the global settings file.

Proposed Solution

• Add a short subsection (EXAMPLES or ARCHITECTURE): “Global defaults vs per-agent overrides”.
• Include a small table: defaults.litellm_model, defaults.timeout, defaults.temperature, defaults.orchestrator_model, and when the wizard overrides them.
• Cross-link rooms.settings.example.yaml, python cli.py config init, and built-in fallbacks when no file exists.
• No code changes required unless a one-line CLI hint on startup is desired (optional, separate commit).

Alternatives Considered

• Rely on example YAML comments only — insufficient; example file is not read by everyone before first run.
• Duplicate full settings schema in README — too heavy; link to example + one cheat-sheet table is enough.

Priority

Low 🟢

Additional Context

Complements #28; no dependency on presets or auto_select_first issues. Good good-first-issue for contributors comfortable with Markdown only.