docs: Comprehensive documentation for Codex CLI & codex-rs (.tangent/)

[tangent-sh]

This PR adds a comprehensive set of developer-centric documentation to .tangent/, covering:

• High-level overview and CLI usage guide
• codex-cli core logic and architecture
• Major React-based CLI components
• Custom hooks for terminal UIs
• Agent, logger, patch, history, and utility internals
• Example projects, prompting, and advanced usage patterns
• Test philosophy & coverage
• codex-rs crate structure and Rust-native features
• Contribution, branching, release procedure, and community values

All docs are cross-referenced and designed for easy onboarding and deep dives.

This should enable rapid onboarding, safe contribution, and correct/secure usage by new developers or users.

Location: All new docs live in .tangent/ and do not interfere with runtime code.

This is an automated documentation overhaul. Ready for review!

[github-actions]

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