[Feature]: Support “Revert to this message” in non-Git workspaces

[kelvinOfoli]

Before submitting

• I searched existing issues and did not find an exact duplicate.
• I am describing a concrete problem or use case, not just a vague idea.

Area

apps/server / apps/web

Problem or use case

The Revert to this message action is unavailable when a project directory is not inside a Git repository. Hovering a completed user message shows only Copy because T3 Code does not create a checkpoint for that turn.

This makes message-level rewind unavailable in legitimate non-Git workspaces, such as prototypes, legacy source folders, generated projects, and projects that have not been initialized as repositories yet.

Steps to reproduce

1. Add a project whose workspace directory is not inside a Git repository.
2. Start a thread and complete two or more turns.
3. Hover over an earlier completed user message.
4. Observe that only the Copy action is shown; Revert to this message is absent.

Expected behavior

Users should be able to return to an earlier message in non-Git workspaces as well.

Ideally, T3 Code would provide app-managed filesystem snapshots for these workspaces so the conversation and workspace can both be restored safely.

At minimum, T3 Code should offer an explicit conversation-only rewind when no filesystem checkpoint backend is available, with clear wording that workspace files will remain unchanged.

Actual behavior

Checkpoint capture exits when the workspace is not a Git repository, and the UI hides the revert action entirely. There is no visible explanation or alternative rewind path.

Suggested behavior

• Introduce a non-Git checkpoint backend using app-managed snapshots, or another recoverable filesystem mechanism.
• If that is out of scope, expose conversation-only rewind as a fallback.
• Explain in the UI why filesystem restoration is unavailable.
• Preserve the existing confirmation step and clearly state whether the operation will restore files, conversation history, or both.

Why this matters

Rewind is a conversation-management feature as well as a source-control feature. Requiring Git makes it unexpectedly disappear and forces users either to initialize a repository solely for T3 checkpoints or abandon the existing thread.

Acceptance criteria

• A completed turn in a non-Git workspace offers a visible rewind option.
• The confirmation dialog accurately describes what will be restored.
• If filesystem snapshots are supported, reverting restores both tracked and newly created workspace files without touching anything outside the configured workspace root.
• If only conversation rewind is possible, workspace contents remain untouched and the UI states that explicitly.

Related

Related but not identical: #5490 requested conversation-only rewind while keeping the current workspace. This issue specifically covers making rewind available when the workspace is not a Git repository.