feat(subagent): show [1m] context-variant suffix on subagent model labels

[tmck-code]

Context

Subagents can run on the 1M-token context-window variant of their model (raw
model id carries a literal [1m] substring, e.g. claude-opus-4-8[1m]), but
the subagent cohort rows had no way to show this — every row displayed just
the model bucket name (opus, sonnet, …), so a 1m-variant subagent looked
identical to a plain one. This PR adds a [1m] suffix to the model label when
the raw model id carries the marker, and threads a cohort-wide shared field
width through subagent_row so mixed cohorts (some [1m], some not) still
line up on a common left edge instead of drifting apart. Ships as v0.4.6.

Changes

New [1m] variant marker

• claude/yas/constants.py: added MODEL_VARIANT_1M = '[1m]', the literal
  substring a subagent's raw model id carries when running the 1M-token
  context variant.
• claude/yas/renderer.py: added subagent_model_label(model), which appends
  MODEL_VARIANT_1M after the model bucket name (model_key) when present —
  the colour bucket itself (model_key/model_colour) is unchanged, only the
  displayed text widens.

Shared cohort-wide model field width

• Renderer.subagent_row gained a model_field_w: int = 6 parameter (default
  matches the historical fixed .rjust(6)), used to right-justify the model
  label in both the one-line and two-line row layouts.
• claude/yas/layout.py: added _cohort_model_field_w(subs), which computes
  the max visible width of every subagent's label in a cohort (floored at 6)
  and threads it into every subagent_row(...) call for that cohort — narrow,
  medium, wide, side-by-side, two-column subagent grid, and two-column
  workflow-agent rows all now share one column edge per cohort instead of each
  row justifying independently.

Tests

• test/test_subagent_rows.py: new test asserting the [1m] suffix renders
  in the two-line row layout.
• test/test_layout_subagent_rows.py: new tests asserting the suffix surfaces
  through build_wide, and that a mixed cohort (opus[1m] beside plain
  sonnet) shares one model-field left edge and width across rows.

Checklist

• Tests added or updated for new/changed behaviour
• (bug fixes) a regression test now covers the situation so it can't come back
• N/A — no behaviour change, because:

Screenshots / recording

before	after

Note: the demo fixtures (ops/demo.py) have no way to force a [1m]-variant
model id onto a synthetic subagent, so these renders exercise the shared
column-width mechanics but not the [1m] suffix text itself — that's
covered by the unit tests above instead.

• N/A — no visible change

Benchmark

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
main	45.7 ± 7.6	40.5	80.3	1.00 ± 0.18
PR	45.5 ± 3.2	40.6	58.1	1.00

• N/A — no performance-relevant change

System info

Key	Value
OS	Linux archlinux 7.1.2-arch3-1 #1 SMP PREEMPT_DYNAMIC Fri, 03 Jul 2026 23:25:36 +0000 x86_64 GNU/Linux
Claude Code	2.1.207 (Claude Code)
Terminal	TERM=tmux-256color TERM_PROGRAM=tmux SHELL=/bin/bash COLORTERM=truecolor
Locale	LANG=en_AU.UTF-8 LC_ALL=
Python	Python 3.13.13
uv	uv 0.11.26 (396ef7ce4 2026-06-30 x86_64-unknown-linux-gnu)