[Bug] Resumed sessions ignore [1m] context spec, fall back to 200k (regression in 2.1.144)

Description

Resumed sessions ignore the [1m] context window suffix in the model setting and fall back to 200k context. Fresh sessions work correctly.

Repro

1. Set "model": "claude-opus-4-6[1m]" in ~/.claude/settings.json
2. Start a fresh session - /context shows 1M (correct)
3. Exit and resume the session - /context shows 200k (wrong)

⛁ ⛁ ⛁ ⛁ ⛁ ⛁ ⛁ ⛁ ⛀ ⛁   Opus 4.6
⛁ ⛁ ⛁ ⛁ ⛁ ⛁ ⛁ ⛁ ⛁ ⛁   claude-opus-4-6
⛁ ⛁ ⛁ ⛁ ⛁ ⛁ ⛁ ⛁ ⛁ ⛁   67.7k/200k tokens (34%)

Regression

• Broken: 2.1.144 (auto-updated 2026-05-19)
• Working: 2.1.143 (downgrading fixes it)

Environment

• macOS, standalone installer
• claude-opus-4-6[1m] in global settings.json