snapshot: fix cwd for git commands in subdirs

When opencode is launched from a git repository subdirectory,
the snapshot service passes file paths relative to the worktree
root but sets cwd to the launch directory. Git cannot resolve
the paths and fails with "did not match any files".

Use the worktree root as cwd so paths resolve correctly
regardless of which subdirectory opencode is started from.

Fix #27688
Close #27737