fix(tui): single plan snapshot and editor history navigation

[xuxujo97-coder]

Summary

Three TUI feedback fixes:

• Editor history navigation — in a non-empty single-line draft, pressing Up once now recalls the most recent history entry instead of only moving the cursor to line start; Down restores the draft. Multi-line drafts keep normal vertical cursor movement.
• Fullscreen transcript navigation — Ctrl+End in fullscreen mode jumps the transcript to the bottom and restores output follow; in regular mode it keeps controlling the input cursor.
• Task plan snapshots — each successful task_update still produces a full, immutable plan snapshot, but the transcript now replaces the previous successful snapshot instead of stacking duplicate full plans. Applies to both live and resumed sessions; failed updates remain visible.

Testing

• pnpm run check passes (formatting, lint, type checks)
• Updated/added unit tests: editor navigation, keybindings, alt-screen key routing, task-plan transcript dedup (live events + session rebuild)
• Manual acceptance via tmux capture-pane on a source-run CLI (140x36): single-line Up/Down recall, multi-line cursor behavior, regular-mode Ctrl+End, fullscreen bottom-jump and follow restore, repeated task_update leaving exactly one full Updated Plan block, session resume keeping only the latest full snapshot

Notes

The task list remains session-scoped and cumulative; plan snapshots render the full task list (existing semantics, unchanged by this PR).