Add same-session conversation rewind

[TY37zhang]

TL;DR

Add reversible same-session conversation rewind so users can edit an earlier prompt without creating a fork. Rewind is available through /revert or double Escape and remains recoverable with /unrevert until continuation.

Issue

• Tangents cannot be removed from the active conversation
• Earlier prompts cannot be reopened for editing

Fix

• Add reversible conversation rewind and prompt selection
• Preserve abandoned turns until the next submission
• Isolate rewound turns from model and permission context