[config] Detect git worktrees for project trust

[dylan-hurd-oai]

Summary

When resolving our current directory as a project, we want to be a little bit more clever:

1. If we're in a sub-directory of a git repo, resolve our project against the root of the git repo
2. If we're in a git worktree, resolve the project against the root of the git repo

Testing

• Added unit tests
• Confirmed locally with a git worktree (the one i was using for this feature)

[github-actions]

Summary
Adds worktree-aware trust resolution: core now treats a linked git worktree as trusted if the main repo is trusted, and TUI trusts the root project when setting trust. Includes unit tests for the resolver.

Review
Nice, cohesive change that aligns core logic and TUI UX around worktrees. A couple small idiomatic tweaks could tighten things up.

• codex-rs/core/src/config.rs: In the trust check, avoid allocations by using map_or(false, |p| p.trust_level.as_deref() == Some("trusted")) instead of clone().unwrap_or("".to_string()) == "trusted".
• codex-rs/core/src/git_info.rs: The .git parser looks good; consider handling multi-line .git content by scanning for the first line starting with gitdir: to be extra robust.
• codex-rs/tui/src/onboarding/trust_directory.rs: Great UX improvement using the resolved root; the error message uses inline variables in format!, consistent with project style.

If not already in the PR body, please add a short motivation: “Trust git worktrees based on the main repo’s trust, so users don’t have to trust each worktree individually.”

Follow-up (outside this PR scope)

• Consider an enum for trust level instead of string comparisons for better type safety.

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[nornagon-openai]

let's just shell out to git for this.

git rev-parse --git-common-dir

[dylan-hurd-oai]

ah, nice

[dylan-hurd-oai]

@nornagon-openai updated 👍

[nornagon-openai]

lgtm with nits