Improve error message when skill/agent frontmatter specifies unsupported model

Problem

When a skill or agent specifies a model in its YAML frontmatter that isn't available in the user's environment, the error message is generic and doesn't indicate the source of the model string:

There's an issue with the selected model (us.anthropic.claude-opus-4-6-v1[1m]). It may not exist or you may not have access to it. Run /model to pick a different model.

This is confusing because the user didn't select that model — it came from a skill's frontmatter. The user has no way to know which skill caused the issue or how to fix it without investigating on their own.

Expected behavior

When the model string originates from a skill or agent frontmatter, the error message should include that context. For example:

Skill "my-skill" specifies model "us.anthropic.claude-opus-4-6-v1[1m]" in its frontmatter, but this model is not available in your environment. Update the skill's model: field to use an abstract model name (sonnet, opus, or haiku), or run /model to pick a different model.

Key information to include:

• Which skill/agent specified the model
• That the model came from frontmatter, not from the user's selection
• Guidance to fix it — update the skill to use abstract names (sonnet, opus, haiku)

Context

• Skills and agents are supposed to use abstract model names (sonnet, opus, haiku, inherit) rather than provider-specific model IDs
• However, nothing prevents a skill author from hard-coding a full model ID like us.anthropic.claude-opus-4-6-v1[1m]
• When this happens and the model isn't available (e.g., because the user is on a different provider or doesn't have access), the current generic error makes it hard to diagnose