Add conversation-forking implementation plan

[sysread]

SYNOPSIS

Planning doc only - no code. Adds docs/dev/in-progress/conversation-forking.md, the implementation plan for git-style conversation forking.

PURPOSE

Conversations are strictly linear today; exploring an alternate direction means destructive delete-from-here or copy-paste. The plan introduces forking with structural sharing: threads own message segments, a conversation is a root-to-node path, destructive edits inside shared history become forks, and a GC sweep reclaims unreachable rows.

DESCRIPTION

• design: explicit per-thread message positions (replaces created_at ordering + its two timestamp forgeries), three thread columns (parent, fork point, hidden), one transcript resolver, per-worker fork treatment (cursor seeding vs full-transcript framing), fork-on-edit w/ hide-and-replace UX
• ordering: M0 QA baselines first, then three zero-behavior-change structural milestones (positions, resolver, hidden+GC), then features (fork primitive + drawer, per-message fork, edit-forks)
• reviewers: the reference-graph audit section explains why GC-time destruction is safe under the existing cascade / set-null / soft-pointer policies

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