fix(opencode): resolve git worktree .git files

[rygwdn]

What does this PR do?

• Read .git files and follow gitdir: references to actual git directory
• Handles both absolute and relative paths
• Falls back gracefully when target doesn't exist

Why?

Git worktrees create a .git file (not directory) containing a path to the actual git directory. Without following this reference, the code fails to store the cached ID, which slows down subsequent startups.

How did you verify your code works?

• Added test for absolute gitdir path resolution
• Added test for relative gitdir path resolution
• Added test for non-existent target fallback
• Tested opening in a worktree, a normal git checkout, and outside of git

[github-actions]

Hey! Your PR title perf: resolve git worktree .git files doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found several potentially related PRs that might be addressing similar git-related issues:

Related PRs (Not Direct Duplicates)

1. PR fix: desktop app worktree collision - use separate ID per worktree #5647 - "fix: desktop app worktree collision - use separate ID per worktree"

   • fix: desktop app worktree collision - use separate ID per worktree #5647
   • Related: Also addresses worktree-specific issues

2. PR fix: file changes visibility in worktree subdirs (#4752) #4817 - "fix: file changes visibility in worktree subdirs"

   • fix: file changes visibility in worktree subdirs (#4752) #4817
   • Related: Addresses worktree subdirectory handling

3. PR Fix relative paths for getGitFiles #2862 - "Fix relative paths for getGitFiles"

   • Fix relative paths for getGitFiles #2862
   • Related: Deals with relative path resolution in git operations

These PRs touch on worktree and git path handling, but none appear to be direct duplicates of PR #7517. Your PR (#7517) specifically addresses the performance issue of resolving .git files that reference git directories via gitdir: paths, which is distinct from the other issues handled in these related PRs.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.