Configurable ANSI palette for custom collaboration modes

[AmirTlinov]

Fixes #9889

Problem

• Custom collaboration presets exist, but default colors are fixed and not user-configurable.

Solution

• Add collaboration_mode_palette (ANSI-only) and allow per-preset color.
• Deterministic default color per preset name + palette; validate non-empty palette.
• Update schema/docs/tests.

Testing

• cargo test -p codex-protocol
• cargo test -p codex-core (flaked once on read_file_tools_run_in_parallel; rerun passed)
• cargo test -p codex-core --test all suite::tool_parallelism::read_file_tools_run_in_parallel

[etraut-openai]

Our contribution guidelines state that we're not accepting feature contributions at this time. We need to make sure that all new features are well designed, compose well with both existing and upcoming features, fit into our roadmap, and are consistent across all Codex surfaces (CLI, IDE extension, web). If you would like to propose a new feature, please file or upvote an enhancement request in the issue tracker. We will generally prioritize new features based on community feedback.