feat: add !model command for runtime model switching via session/set_model (Kiro backend)

[sh14y]

Summary

Add a !model Discord command that lets users list and switch the underlying LLM model for the current ACP session at runtime, without restarting the bot or recreating the session.

Motivation

Kiro CLI's ACP server already exposes multiple models (Claude Opus 4.6 / Sonnet 4.6 / Haiku 4.5 / Auto). Today openab pins whatever Kiro chose at session/new time and there is no in-Discord way to change it. Different tasks benefit from different models (e.g. Opus for hard reasoning, Haiku for cheap quick replies, Auto when you don't care). Letting users switch in-thread keeps the workflow inside Discord.

ACP protocol verification

Verified against Kiro CLI v1.29.3:

1. session/new response now includes a models object:

   {
     "result": {
       "sessionId": "...",
       "models": {
         "currentModelId": "auto",
         "availableModels": [
           { "modelId": "auto",               "name": "Auto",      "description": "..." },
           { "modelId": "claude-opus-4.6",    "name": "Opus 4.6",  "description": "..." },
           { "modelId": "claude-sonnet-4.6",  "name": "Sonnet 4.6","description": "..." },
           { "modelId": "claude-haiku-4.5",   "name": "Haiku 4.5", "description": "..." }
         ]
       }
     }
   }

2. session/set_model JSON-RPC method accepted:

   { "method": "session/set_model",
     "params": { "sessionId": "...", "modelId": "claude-opus-4.6" } }
   → { "result": {} }

Proposed command design

Inside an allowed channel/thread:

Input	Behavior
!model	List all available models, marking the current one
!model opus	Switch to claude-opus-4.6
!model sonnet	Switch to claude-sonnet-4.6
!model haiku	Switch to claude-haiku-4.5
!model auto	Switch back to auto-routing
!model <id>	Accept any exact modelId from availableModels
!model <unknown>	Reply with error + available list

Aliases (opus, sonnet, haiku, auto) are resolved client-side; otherwise the exact modelId is forwarded.

Models marked [Deprecated] or [Internal] in their description are filtered out of the list.

Implementation outline

• New ModelInfo { model_id, name, description } in src/acp/connection.rs
• AcpConnection gains current_model: String and available_models: Vec<ModelInfo>
• session_new() parses the models block from the response
• New AcpConnection::session_set_model(model_id) and resolve_model_alias(input)
• Handler::message() intercepts messages whose prompt starts with !model and dispatches via pool.with_connection

Backend scope

This is currently wired against the Kiro ACP backend (which is the only one I've validated session/set_model on). Other backends that don't implement the method will simply error on switch and report _(no models reported by agent — backend may not support model switching)_ when listing.

PR with the implementation will follow this issue.

[CHC-Agent]

Hi @sh14y,

ACP protocol verification looks solid (session/new models + session/set_model on Kiro CLI v1.29.3). PR #186 is already up with the /model slash command implementation.

Note: other backends (Claude Code, Codex, Gemini) may not support session/set_model — the graceful fallback approach in the issue makes sense.