Conversation-only rewind ("Edit from here") in non-Git projects

[Conte777]

Problem

In a project whose directory is not a Git repository, Edit from here never appears, so there is no way to rewind the conversation to an earlier message.

#10365 asked for this and was closed as a duplicate of #6882. #6882 was resolved by #11358 (Revert and keep changes), but that conversation-only rewind is still gated on a Git checkpoint, so non-Git projects did not get it.

Why it is missing today (main @ 76d3c96)

1. Server: in a non-Git directory every run checkpoint is recorded with status: "missing" (apps/server/src/orchestration-v2/CheckpointService.ts:374-387).
2. Web: deriveRevertTurnCountByUserMessageId only maps user messages whose run has a ready checkpoint (apps/web/src/session-logic.ts:1010-1032), and the button renders only when revertTurnCount is set (apps/web/src/components/chat/MessagesTimeline.tsx:2285).
3. Server: CheckpointRollbackService rejects any target whose checkpoint is not ready with rollback-target-invalid, even when restoreFiles === false (apps/server/src/orchestration-v2/CheckpointRollbackService.ts:131-145).

The checkpoint record itself (run id, ordinalWithinScope) exists for every run; only the Git snapshot is absent. A conversation-only rewind does not read the snapshot.

Proposed change

Allow conversation-only rewind to target a checkpoint in missing status:

• Web: offer Edit from here for user messages whose run has a ready or missing checkpoint, when the provider supports conversation rollback. For a missing checkpoint, show only Revert and keep changes (no file-restore option), with a note that the project is not under Git.
• Server: in CheckpointRollbackService, accept a missing checkpoint when restoreFiles === false; keep requiring ready when files are restored.

Out of scope

File restore without Git. Non-Git projects keep their files as they are, the same as Revert and keep changes already does in Git projects.

Risks

• The workspace may not match the earlier conversation. This is the same tradeoff [Feature]: Allow rewinding a conversation without restoring the workspace #6882 accepted for Revert and keep changes.
• A checkpoint in error status (Git capture failed) should stay excluded, so a broken Git repo does not silently drop the file-restore option.