fix(coding-agent/eval): remove per-call model override from agent() (#6438)

Completes the maintainer's removal of per-call model selection from
subagent spawns (9f8aa87dbf removed it from the task tool and the
model-facing agent() docs/prompt, but the eval agent() runtime and all
four preludes still accepted and forwarded a per-call model).

Subagents now always resolve through the selected agent's frontmatter
model and settings, so an explicit model: "default" can no longer
silently route children onto the parent session model.

- agent-bridge: drops "model?" from agentArgsSchema and the request
  forward; adds "+": "delete" so a legacy model argument is stripped
  (same contract as the task wire schemas).
- JS/Python/Ruby/Julia preludes: remove the model parameter from
  agent(); completion()'s tier selector is unchanged.
- docs (tools/eval.md, python-repl.md) updated to the removed surface.

Refs #6438

packages/coding-agent/src/eval/agent-bridge.ts

@@ -23,7 +23,6 @@ export const EVAL_AGENT_BRIDGE_NAME = "__agent__";
 const agentArgsSchema = type({
 	prompt: "string>0",
 	"agent?": "string>0",
-	"model?": "string>0|string>0[]",
 	"label?": "string",
 	"schema?": "unknown",
 	"schemaMode?": "'permissive' | 'strict'",
@@ -31,12 +30,12 @@ const agentArgsSchema = type({
 	"apply?": "boolean",
 	"merge?": "boolean",
 	"handle?": "boolean",
+	"+": "delete",
 });
 
 interface EvalAgentArgs {
 	prompt: string;
 	agent?: string;
-	model?: string | string[];
 	label?: string;
 	schema?: unknown;
 	schemaMode?: StructuredSubagentSchemaMode;
@@ -148,7 +147,6 @@ export async function runEvalAgent(args: unknown, options: EvalAgentBridgeOption
 					invocationKind: "eval",
 					assignment: parsed.prompt,
 					...(parsed.agent !== undefined ? { agent: parsed.agent } : {}),
-					...(parsed.model !== undefined ? { model: parsed.model } : {}),
 					...(Object.hasOwn(parsed, "schema") ? { outputSchema: parsed.schema } : {}),
 					...(parsed.schemaMode !== undefined ? { schemaMode: parsed.schemaMode } : {}),
 					...(parsed.label !== undefined ? { identity: { label: parsed.label } } : {}),

packages/coding-agent/src/eval/jl/prelude.jl

@@ -519,14 +519,11 @@ function completion(prompt::String; model="default", system=nothing, schema=noth
     return schema === nothing ? text : Main.json_parse(string(text))
 end
 
-function agent(prompt::String; agent="task", model=nothing, label=nothing, schema=nothing, schema_mode=nothing, isolated=nothing, apply=nothing, merge=nothing, handle=false, kwargs...)
+function agent(prompt::String; agent="task", label=nothing, schema=nothing, schema_mode=nothing, isolated=nothing, apply=nothing, merge=nothing, handle=false, kwargs...)
     args_dict = Dict{String, Any}("prompt" => prompt)
     if agent !== nothing
         args_dict["agent"] = agent
     end
-    if model !== nothing
-        args_dict["model"] = model
-    end
     if label !== nothing
         args_dict["label"] = label
     end

packages/coding-agent/src/eval/js/shared/prelude.txt

@@ -104,8 +104,8 @@ if (!globalThis.__omp_js_prelude_loaded__) {
 			"agent",
 			opts,
 			rest,
-			["agent", "model", "label", "schema", "isolated", "apply", "merge", "schemaMode"],
-			"{ agent, model, label, schema, isolated, apply, merge, schemaMode, handle }",
+			["agent", "label", "schema", "isolated", "apply", "merge", "schemaMode"],
+			"{ agent, label, schema, isolated, apply, merge, schemaMode, handle }",
 		);
 		const { handle, ...callArgs } = o;
 		const res = await globalThis.__omp_call_tool__("__agent__", { prompt, ...callArgs, handle: Boolean(handle) });

packages/coding-agent/src/eval/py/prelude.py

@@ -488,7 +488,6 @@ if "__omp_prelude_loaded__" not in globals():
         prompt,
         *,
         agent="task",
-        model=None,
         label=None,
         schema=None,
         schema_mode=None,
@@ -506,8 +505,6 @@ if "__omp_prelude_loaded__" not in globals():
         args = {"prompt": prompt}
         if agent is not None:
             args["agent"] = agent
-        if model is not None:
-            args["model"] = model
         if label is not None:
             args["label"] = label
         if schema is not None:

packages/coding-agent/src/eval/rb/prelude.rb

@@ -392,10 +392,9 @@ unless defined?($__omp_prelude_loaded) && $__omp_prelude_loaded
     schema.nil? ? text : JSON.parse(text)
   end
 
-  def agent(prompt, agent: "task", model: nil, label: nil, schema: nil, schema_mode: nil, isolated: nil, apply: nil, merge: nil, handle: false)
+  def agent(prompt, agent: "task", label: nil, schema: nil, schema_mode: nil, isolated: nil, apply: nil, merge: nil, handle: false)
     args = { "prompt" => prompt }
     args["agent"] = agent unless agent.nil?
-    args["model"] = model unless model.nil?
     args["label"] = label unless label.nil?
     args["schema"] = schema unless schema.nil?
     args["schemaMode"] = schema_mode unless schema_mode.nil?