fix(opencode): resolve snapshots from git subdirectories

[MuoDoo]

Issue for this PR

Closes #27688

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

When opencode is launched from a git subdirectory, snapshot candidate paths can be worktree-relative while later pathspec operations run from the launch directory. This can make git resolve paths like src/file.txt under src/src/file.txt and fail to record the snapshot.

This PR resolves pathspec-consuming snapshot operations from the worktree root, makes untracked file paths worktree-relative, and adds a regression test for launching from a subdirectory.

Related prior attempts: #27737, #28131.

How did you verify your code works?

• bun test test/snapshot/snapshot.test.ts --timeout 30000
• bun typecheck

Screenshots / recordings

N/A, not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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

I found potential related PRs that address similar snapshot/git subdirectory issues:

1. PR snapshot: fix cwd for git commands in subdirs #28131 - snapshot: fix cwd for git commands in subdirs

   • Related to fixing git command execution in subdirectories

2. PR fix(snapshot): run git from worktree, not launch directory #27737 - fix(snapshot): run git from worktree, not launch directory

   • Directly addresses running git from the correct worktree location

These PRs appear to address similar or overlapping concerns about resolving git operations from the correct directory context when working in subdirectories. You may want to review these to ensure there's no duplication of effort.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍