[BUG] Team agent teammates don't inherit [1m] context window variant from leader (v2.1.80)

Bug Description

Team agent teammates (spawned via TeamCreate + Agent with team_name) inherit the leader's model ID (claude-opus-4-6) but NOT the [1m] context window variant. The leader runs on claude-opus-4-6[1m] (1M context), but spawned teammates resolve to claude-opus-4-6 (200k context).

This is distinct from #32368 (model ID inheritance, fixed in v2.1.72) — the model ID inherits correctly, but the context window modifier is stripped.

Steps to Reproduce

1. Start Claude Code on Max plan — /model shows "Default (recommended) — Opus 4.6 with 1M context" selected
2. Configure ~/.claude/settings.json:

{
  "model": "opus[1m]",
  "env": {
    "CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS": "1",
    "CLAUDE_CODE_SUBAGENT_MODEL": "opus[1m]",
    "ANTHROPIC_DEFAULT_OPUS_MODEL": "claude-opus-4-6[1m]"
  }
}

1. Create a team: TeamCreate
2. Spawn a teammate: Agent with team_name parameter
3. In the teammate's pane, run /model

Expected Behavior

Teammate's /model shows option 1: "Default (recommended) — Opus 4.6 with 1M context" ✔

Actual Behavior

Teammate's /model shows option 5: "Opus 4.6 — claude-opus-4-6" selected (200k, no 1M)

The teammate's system prompt confirms: "You are powered by the model named Opus 4.6. The exact model ID is claude-opus-4-6." — no [1m] suffix.

Notes

• Standalone subagents (no team_name) DO correctly inherit opus[1m] via CLAUDE_CODE_SUBAGENT_MODEL — their system prompt shows claude-opus-4-6[1m]
• Team teammates spawned via TeamCreate + Agent(team_name=...) do NOT pick up CLAUDE_CODE_SUBAGENT_MODEL, model, or ANTHROPIC_DEFAULT_OPUS_MODEL for the [1m] variant
• The model ID itself inherits correctly (teammates get claude-opus-4-6, not a different model) — only the [1m] context modifier is lost
• v2.1.72 changelog says "Fixed team agents to inherit the leader's model" — this fixed the model ID but not the context window variant

Environment

• Claude Code: v2.1.80
• Plan: Claude Max
• OS: macOS (Darwin 25.3.0, M5 Max)
• Terminal: iTerm2 with tmux teammate mode
• Leader model: claude-opus-4-6[1m] (confirmed via /model → option 1)

Related Issues

• #32368 — Team agents model inheritance (fixed in v2.1.72, but [1m] variant still stripped)
• #29231 — Task tool subagent spawning ignores model context window variant
• #33154 — Cowork forces claude-opus-4-6[1m] causing rate limits