Bug: `/undo` reports success but does not revert files in non-Git projects

Description

Bug Description

When using OpenCode in a non-Git project directory, running /undo or clicking the undo button shows "1 message reverted" but the actual file changes are NOT reverted at all.

Steps to Reproduce

1. Start OpenCode in a non-Git directory (no git init was run)
2. Ask OpenCode to modify a file (e.g., edit or write content)
3. Run /undo or click the undo button
4. Observe that it shows a success message, but the file content remains unchanged

Root Cause

OpenCode's undo mechanism relies on Git snapshots for file change tracking. In snapshot/index.ts, when state.vcs !== "git", the snapshot system is silently disabled — track() returns undefined and revert()/restore() become no-ops. However, the higher-level undo flow does not check this state and still reports "success" to the user.

Environment

• OpenCode version: 1.18.4
• OS: Windows 11 (Build 26200)
• PowerShell: 5.1.26100.8115

Expected Behavior

When the project is not a Git repository, the undo action should clearly inform the user, for example: "Cannot undo file changes: the current project is not a Git repository. Please use this feature in a Git project."

Additional Suggestion (Feature Request)

Projects without Git actually have a greater need for undo functionality — users in Git projects can always manually recover via git checkout or git restore, but non-Git projects have no way to restore once files are modified. Consider implementing a file-copy or content-hash-based snapshot mechanism for non-Git environments in the future.

Plugins

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OpenCode version

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Steps to reproduce

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Screenshot and/or share link

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Operating System

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Terminal

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