feat(agent-core-v2): advertise model capabilities in the subagent model picker

[7Sageer]

Related Issue

No linked issue — the problem is explained below.

Problem

With the secondary-model experiment enabled, the subagent collaboration tools (Agent / AgentSwarm) advertise the routable models in their model parameter descriptions — but only as bare model ids. The parent agent has no way to tell whether the secondary model supports images or thinking, so it can delegate a multimodal task to a text-only model; the engine then silently degrades (media tools unregistered, images replaced with placeholder text) without the parent ever knowing.

What changed

Each advertised model line now carries a capabilities: ... suffix derived from the model catalog's resolved capability flags (image_in, video_in, audio_in, thinking, tool_use, dynamically_loaded_tools), e.g.:

- secondary: example-provider/example-model (default) — the configured secondary model; prefer it for routine subagent tasks; capabilities: image_in, thinking, tool_use

• The lookup follows the same binding rule as spawn time: a secondary model configured via a patch recipe resolves capabilities from the derived entry the subagent would actually run on.
• Fail-soft: a model that fails catalog resolution (dangling recipe pointer, unconfigured entry) silently omits the suffix instead of breaking tool-schema generation; spawn still surfaces the real error.
• Numeric limits (max_context_tokens) are deliberately excluded — only the boolean ability array is advertised.
• No changeset: the advertised lines only exist while the secondary-model experiment flag (default off) is enabled, so nothing changes on shipped surfaces.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

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