Feature Request: Support per-worker model in swarm spawn

Problem

Current behavior:

• /swarm spawn "agent" model=some-model ignores the model= parameter
• All spawned workers use swarm_model from config.toml
• This makes it impossible to assign different models to different agents

Expected behavior:

• /swarm spawn "agent" model=some-model should use some-model for that worker
• swarm_model should only be used as fallback when model= is not specified

Use Case

We have 6 agents with different model requirements (v6.2 binding table):

Current limitation forces all agents to use agnes-2.5-flash, wasting free models and reducing quality for code/review tasks.

Proposed Solution

Modify resolve_swarm_spawn_selection to:

1. Check if model= parameter is explicitly provided
2. If yes, use the provided model
3. If no, fallback to swarm_model from config

References

• Issue #1156: "model= parameter being ignored is expected"
• Commit b76f96561: fix(swarm): make worker model operator-controlled

Locate resolve_swarm_spawn_selection and the /swarm spawn entry point, then inspect how config.toml supplies swarm_model. Verify that an explicit model= value is used for that worker while swarm_model remains the fallback when model= is omitted; run the relevant existing swarm tests if present.