Feature: Version-pinned model parameter in Agent tool

Feature: Version-pinned model parameter in Agent tool

Current behavior

The Agent tool's model parameter only accepts generic names: "sonnet", "opus", "haiku". These resolve to the current versions at runtime.

Requested behavior

Allow specific model IDs: "claude-opus-4-5-20251101", "claude-sonnet-4-6", etc.

Use case

Multi-agent skills/plugins that spawn agents need version stability. When a new model version ships, all spawned agents silently upgrade — breaking prompts tuned for specific model behavior.

Example: a skill using model: "opus" for synthesis today gets Opus 4.7. Tomorrow it might get 4.8 with different reasoning patterns, breaking the aggregation logic.

Proposal

Accept both formats:

• model: "sonnet" → latest (current behavior)
• model: "claude-sonnet-4-6" → pinned version