[BUG] Agent Teams teammates lose [1m] context window suffix — root cause: tmux spawn hardcodes model without variant

Preflight Checklist

• [x] I have searched existing issues and found related closed issues (#24429, #27038, #27261) but they were closed as NOT_PLANNED without addressing this specific root cause
• [x] I can reproduce this on the latest version of Claude Code

Bug Description

When spawning Agent Teams teammates, the [1m] context window variant suffix is stripped from the --model argument passed to the tmux pane process. This silently downgrades teammates from 1M to 200K context window.

Root Cause (verified via ps -eo pid,args)

The parent session runs as claude-opus-4-6[1m], but when spawning a teammate via tmux, the CLI constructs the command as:

/Users/.../.local/share/claude/versions/2.1.76 \
  --agent-id inspect-me@test-team-9 \
  --agent-name inspect-me \
  --team-name test-team-9 \
  --agent-color blue \
  --parent-session-id 192a86de-... \
  --model claude-opus-4-6          ← [1m] suffix stripped

The --model value is claude-opus-4-6 instead of claude-opus-4-6[1m]. The variant suffix is lost during model resolution before the tmux command is constructed.

Reproduction

# Parent session: claude-opus-4-6[1m]

# Test 1: Subagent (inherits [1m] correctly)
Agent(prompt="Report your model ID")
→ "claude-opus-4-6[1m]" ✅

# Test 2: Teammate (loses [1m])
TeamCreate(team_name="test")
Agent(team_name="test", name="t1", prompt="Report your model ID")
→ "claude-opus-4-6" ❌ (200K instead of 1M)

# Test 3: Subagent spawned FROM a teammate (inherits [1m])
# (teammate spawns Agent() without team_name)
→ "claude-opus-4-6[1m]" ✅

Test 3 is particularly interesting — subagents always inherit [1m] regardless of where they're spawned from, but the teammate itself never gets it.

Impact

• Silent 5x context reduction: Teammates intended for large codebase analysis (research, architecture review, module exploration) are silently limited to 200K instead of 1M
• No workaround: The model parameter enum only accepts "sonnet" | "opus" | "haiku" — cannot pass "opus[1m]". Setting model: "opus[1m]" in settings.json also does not propagate to teammates
• Cost without benefit: Users paying for 1M context don't get it on teammates

Suggested Fix

In the teammate spawn path (tmux/iTerm2 backend), preserve the full model string including variant suffix when constructing the --model argument:

- --model claude-opus-4-6
+ --model claude-opus-4-6[1m]

Or better: if no explicit model override is specified for the teammate, don't pass --model at all and let it inherit from the session default (same as subagents do).

Environment

• Claude Code version: 2.1.76
• OS: macOS Darwin 24.6.0
• Model: claude-opus-4-6[1m]
• Teammate backend: tmux (pane-based)

Related Issues

• #24429 — Root issue for model parameter ignored (closed NOT_PLANNED)
• #27038 — Detailed source analysis of the bug
• #27261 — Duplicate focusing on [1m] inheritance