tui onboarding: warn when trusting git subdirectories

Summary

When a user runs Codex from a subdirectory within a Git repository and chooses to trust the directory, it's unclear that the trust decision applies to the repository root, not just the current directory.

Proposal

Add a yellow warning message in the TUI onboarding screen that clarifies when the user is in a Git subdirectory, showing which directory will actually be trusted.

Implementation

The fix has been implemented and tested at: https://github.com/joycebeatriz/codex/pull/1

• Added conditional warning message when cwd != trust_target
• Updated snapshot tests
• Included Contributor Covenant v3.0 adoption