feature(fork session): introduce fork session feature with <leader>f or f in session timeline modal

[spoons-and-mirrors]

Summary

Fork the active session into a brand‑new one.
The fork copies existing messages (and their parts) into a new session titled: Fork of <original title>

Usage

1. Full fork: <leader>f

   • Copies the visible conversation.

2. Partial fork from a point:

   • Open the timeline: <leader>g
   • Move to a user message (↑/↓)
   • Press f
   • Copies session up to the selected message (inclusive, with assistant response)

Typical Uses

• Explore an alternative approach without losing current progress.
• Branch before a long assistant/tool sequence and try a different prompt.
• Save a clean starting point after pruning via revert, then iterate separately.

Notes

• Does not mutate the source session.
• Forking during an in‑progress assistant response only captures what has completed so far. (source session streaming is not interupted)

[ofonseca-pyming]

is possible to merge this? @rekram1-node

[rekram1-node]

@extrememicro we have forking functionality in the rewrite branch that we are about to ship (next week) along with a bunch of other improvements

i would need to review the code for this before merging but all the go code is going to be deleted soon