Side-by-side task checklist & subagent cohort in the wide layout

[tmck-code]

Context

YAS can take up a lot of vertical space if there is a plan/task list and subagents at the same time.

This is the first of a few upcoming PRs that attempt to consolidate the current layout to save on vertical space, which still displaying the same amount of information

Changes

Design / spec: openspec/changes/archive/2026-06-07-side-by-side-tasks-subagents/ (proposal, design, specs).

• Side-by-side composition (wide only): when both the task checklist and ≥1 visible subagent are present and the terminal is wide enough, build_wide renders the checklist as a left column and the subagent cohort as a right column, separated by a single gradient │ divider, instead of stacking them.
• Content-driven split: left column sized to its content up to 45% of inner width; the subagent column takes the remainder. If the remainder would be < 40 cols, it falls back to today's stacked rendering.

Additional changes (verbose)

• Height reconciliation: columns rendered independently to their own widths, then zipped top-aligned to the taller column, padding the shorter with blanks so the divider runs straight.
• Divider elbows: border_separator gains downs support (mirroring border_separator_dim), so the static→dynamic seam grows a ┬ at the divider column and the separator below grows a matching ┴.
• Subagent two-line row redesign (all wide uses): the ▶/✓ marker is removed; elapsed duration moves to the front of line 1; right cluster becomes share% · tok · model (sheds share% → tok under width pressure, always keeps model + duration); line 2 becomes activity-only. The t/m rate and ↑output fields are removed.
• Subagent one-line row: ↑output field removed; otherwise unchanged.
• Builder-driven geometry: task_row and subagent_row take an explicit content-width; the two-line/one-line form becomes a builder-supplied flag rather than a width > 100 self-decision, so a narrow side-by-side column can still use the two-line form.

Checklist

• Tests added or updated for new/changed behaviour

Screenshots / recording

state	screenshot
before
after

Benchmark

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
main	61.5 ± 4.3	58.1	79.5	1.00
PR	62.5 ± 5.3	59.4	93.7	1.02 ± 0.11

System info

Key	Value
OS	Linux archlinux 7.0.10-arch1-1 #1 SMP PREEMPT_DYNAMIC Sat, 23 May 2026 14:21:20 +0000 x86_64 GNU/Linux
Claude Code	2.1.168 (Claude Code)
Terminal	TERM=tmux-256color TERM_PROGRAM=tmux SHELL=/bin/bash COLORTERM=truecolor
Locale	LANG=en_AU.UTF-8 LC_ALL=
Python	Python 3.13.1
uv	uv 0.11.14 (x86_64-unknown-linux-gnu)