Thread TUI roadmap

Turns the thread reader into an email-style, branching conversation view.

Locked design

Newest-on-top, flat. Messages render newest-first (latest at the top, no scrolling to reach it). Reading down goes older / toward the root.
↳ re: tags, no depth indent. Each message tags its parent (↳ re: <preview>, which sits below it). Indentation is reserved only for distinguishing forked branches.
Collapsed by default. First-line preview + [N ▸] on nodes with hidden descendants; arrow to expand; full-view for any message.
Timestamps to the minute: [MM-DD, HH:MM].
Reply at cursor. w on any message composes a reply under that node. The deliberation context is the root→node path (to_deliberation_query_from).
Per-branch session identity. branch_id is the conversation_id. Replying to a leaf continues that branch (session resume); replying under a non-leaf node forks → fresh branch_id/conversation_id, cold-seeded from the path (claude-code resume is linear, so forks replay context).
Merged inbox. Subject list + thread reader fold into one screen.

Data model. A Message carries id, parent_id, and branch_id; a Thread exposes get, children, is_leaf, path_to_root, tip, and reply. to_deliberation_query_from(parent) builds the root→node context, migrate_linear upgrades a flat transcript on load, and push_message linear-appends with auto-linking.
Reply-at-cursor. Composing a child under the selected node carries the parent id + branch into the deliberation (conversation_id = branch_id). A top-level compose starts a new root branch; replying under a non-leaf node is detected as a fork. The user turn is appended via Thread::reply.
Collapsible newest-on-top reader. The transcript renders newest-first with ↳ re: tags, [N ▸] fold counts, timestamps, and fork indent by fork depth (⑂N marker at fork points). Rows expand/collapse (→/←/Enter), collapsed rows drop trailing blanks so End reaches the oldest message, and ▲/▼ indicators show items above/below the viewport.
Full-message view. o opens the selected message full-screen (header + scrollable content); ↑↓/PgUp/PgDn scroll, Esc/q return.
New-root turn. n starts a fresh line under the subject, rooted at nothing.
Merged subject+thread inbox. Inbox rows sit above a live peek pane that shows the selected thread's newest turns below the list, so the latest is visible on one screen without opening it. Push-navigation moves from the list into the reader.