fix(snapshot): use worktree root as cwd for git operations

[SpencerJung]

Fixes #27688. Changes cwd from state.directory to state.worktree in ignore, drop, and stage functions so that git commands resolve paths correctly when opencode is opened from a subdirectory.

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[github-actions]

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

I found potential duplicate PRs related to this issue:

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

   • fix(snapshot): run git from worktree, not launch directory #27737
   • Directly addresses the same issue of running git operations from the worktree root instead of a different directory

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

   • snapshot: fix cwd for git commands in subdirs #28131
   • Directly addresses the same problem of fixing git command working directory when opened from subdirectories

Both of these PRs appear to be addressing the exact same issue (#27688) related to git operations and worktree/subdirectory path resolution.

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