Skill `model:` pin inherits parent 1M context flag, blocking on Sonnet without /extra-usage

Environment

• Claude Code v2.1.111+ on macOS 15.6 (Darwin 25.3.0)
• Max plan
• Parent session model: claude-opus-4-7[1m] (1M context auto-included free on Max)

Repro

1. Start a session on Opus 4.7 (1M context active by default on Max)
2. Invoke any skill with model: sonnet in frontmatter (e.g. self-improving-agent)
3. Skill loads successfully (Successfully loaded skill · claude-sonnet-4-6) then immediately errors

Actual

API Error: Extra usage is required for 1M context · run /extra-usage to enable, or /model to switch to standard context

Skill is unusable until the user enables paid extra-usage or downgrades the whole session out of 1M mode.

Expected
A skill pinning model: sonnet should run on Sonnet's standard 200K context. The parent session's 1M flag is an Opus-tier free benefit and shouldn't carry over to a different model where it costs extra.

Impact

• Every sonnet-pinned skill on a Max plan with Opus 4.7 default is broken until users discover this and either downgrade context or enable extra-usage.
• Internal guidance recommending model: sonnet for status/audit skills conflicts with default Max plan behavior.
• Workaround (use agent: to fork to a subagent instead) sidesteps the bug but isn't the right fix for skills that genuinely should run inline.

Suggested fix
Treat the 1M context flag as model-scoped, not session-scoped. When a skill or subagent switches the active model, drop the 1M flag unless the new model also includes it free.