Unrevert from subdirectory overwrites files in other directories with stale versions

[ryanwyler]

Description

When running opencode from a subdirectory and clicking "undo revert", files in other directories are overwritten with stale versions (potentially weeks old).

Steps to Reproduce

1. Create a project with multiple subdirectories:

mkdir -p /tmp/testproject/frontend /tmp/testproject/backend
cd /tmp/testproject
git init && git config user.email "[email protected]" && git config user.name "Test"
echo 'console.log("frontend")' > frontend/app.ts
echo 'println("backend ORIGINAL")' > backend/server.go
git add . && git commit -m "Initial"

2. Run opencode from backend/, make a change, then exit:

cd /tmp/testproject/backend
opencode
# Ask it to modify server.go, then exit

3. Update backend externally (simulates time passing):

echo 'println("backend CURRENT")' > backend/server.go
git add . && git commit -m "Update backend"

4. Run opencode from frontend/, make a change, revert, then unrevert:

cd /tmp/testproject/frontend
opencode
# Ask it to modify app.ts
# Click "Revert" on the assistant's message
# Click "Undo Revert"

5. Check if backend was corrupted:

cat /tmp/testproject/backend/server.go
# Shows "backend ORIGINAL" instead of "backend CURRENT"

Expected Behavior

Files in other directories should NOT be affected when running revert/unrevert from a subdirectory.

Actual Behavior

Files in other directories are overwritten with stale versions from the shared snapshot index.

Root Cause

1. The snapshot index at ~/.local/share/opencode/snapshot/{project_id}/ is shared across all sessions in a project
2. git add . from a subdirectory only updates that subdirectory's entries in the index
3. restore() uses checkout-index -a which restores ALL files in the index, including stale entries from other directories
4. diff() uses cwd(Instance.worktree) showing diffs for unrelated directories

Environment

• OpenCode version: v1.1.13
• OS: Linux