docs(rfd): Session Rewind

[htahaozlu]

This RFD proposes adding session/rewind and session/edit_prompt methods to ACP, enabling user-facing conversation rewind and message editing — features universally present in coding-agent UIs (Claude Desktop, Cursor, Windsurf, Gemini CLI checkpointing) but currently lacking a portable protocol surface.

Builds on unstable_message_id (already in Cargo.toml's unstable set).

Complementary to, and distinct from, session/fork: fork is non-destructive context copying for side queries; rewind mutates the session timeline in place.

Consolidates two previously scattered discussions:

• #239 — Restoring a checkpoint and an earlier point in the conversation
• #329 — session/undo and session/redo requests

Filesystem rollback is explicitly out of scope (see Status quo and FAQ in the RFD body) — left to existing fs/* methods, session/fork, IDE-native snapshots, or the user's SCM.

Concrete user-facing bugs in Zed that motivate a portable protocol surface:

• zed#52153 — ACP with Codex CLI hangs: "failed to get old checkpoint — not implemented yet"
• zed#39997 — Zed removes all AI changes when editing a message with no checkpoints consideration
• zed#28676 — Agent Panel: Undo/Redo around checkpoints, checkpoint restore irreversibly destructive
• zed#55888 — Claude Code ACP: conversation after /clear is lost on Zed restart

Status: Draft proposal, open for champion from the core team. Happy to iterate on the design — open to splitting into separate RFDs (rewind vs edit-prompt), merging with related work, or scoping down to a single primitive if preferred.

CLA: will sign at https://zed.dev/cla before any implementation PR. Posting this RFD PR per the RFD process so the design discussion can happen against concrete text.

[htahaozlu]

@cla-bot check

[htahaozlu]

@cla-bot check