fix(coding-agent): clamped auto thinking to undefined for models without controllable effort

Devin provider models (devin-agent) advertise reasoning: true but no
thinking.efforts metadata — Cascade selects effort by routing to sibling
model ids, not a wire param. getSupportedEfforts(model) therefore returns
[]. clampAutoThinkingEffort previously short-circuited that empty supported
list by returning the requested effort as-is, so the auto-thinking
classifier-resolved level (e.g. low) reached stream.ts:1163 where
requireSupportedEffort threw 'Thinking effort low is not supported by
devin/<id>. Supported efforts: '. In --print mode the user saw the error
text; in the TUI it was silently swallowed, producing the reported
'working then empty response' symptom.

Returns undefined when supported is empty so the result mirrors
clampThinkingLevelForModel's behavior on the same shape (the explicit
--thinking low / high paths already worked because of this). Updates
classifyDifficulty's return type to Effort | undefined and threads through
to the existing #applyAutoThinkingLevel undefined-effort early-return.
#applyAutoThinkingLevel also short-circuits the classifier call up front
for these models — there is no effort to pick.

Fixes #3356

packages/coding-agent/src/session/agent-session.ts

@@ -7367,6 +7367,10 @@ export class AgentSession {
 	async #applyAutoThinkingLevel(promptText: string, generation: number): Promise<void> {
 		const model = this.model;
 		if (!model?.reasoning) return;
+		// Models with reasoning but no controllable effort surface (devin-agent
+		// Cascade routes effort via sibling model ids, not a wire param) have
+		// nothing to pick — skip classification rather than discard its result.
+		if (getSupportedEfforts(model).length === 0) return;
 
 		let resolved: Effort | undefined;
 		if (this.#magicKeywordEnabled("ultrathink") && containsUltrathink(promptText)) {