Fork from point in chat buffer (f key)

[dnouri]

What

Press f on any user turn in the chat buffer to fork the conversation from that point. Confirms with a preview, then creates a new session branching from that message.

Also fixes n/p navigation — the old pattern searched for the never-present You: instead of setext headings — and recenters the heading to the top of the window.

How it works

1. Detect which user turn point is in via setext heading scan
2. Fetch the conversation tree (get_tree RPC)
3. Walk from leaf to root to find user message IDs on the active branch
4. Align visible buffer headings to tree IDs using a last-N strategy (handles compaction — compacted-away turns aren't rendered but exist in the tree)
5. Confirm with y-or-n-p, then fork via the existing fork RPC

Upstream enhancements that would simplify this

The main complexity here is mapping a buffer position to an entry ID. We render user turns from get_messages, but forking requires an entry ID from get_tree — two different RPCs with no shared identifiers. This forces us to:

• Fetch the full tree and walk it leaf-to-root
• Count visible headings and align them to tree user IDs via last-N (to handle compaction)

Two possible upstream improvements:

1. Include entryId in get_messages responses. If each message carried its entry ID, fork-at-point would be a single lookup — no tree walk, no alignment heuristic.

2. A fork_at_index RPC that accepts a 0-based user turn index (from the active branch) instead of an entry ID. The server already knows the active branch and could resolve the index internally.

Either would eliminate flatten-tree, active-branch-user-ids, resolve-fork-entry, and the last-N alignment logic (~50 lines of production code and ~20 tests).