fix(coding-agent): preserved explicit :auto suffix in modelRoles

The model selector's persistence path dropped the `:auto` selector when parsing role values, producing a warning ('Invalid thinking level "auto"') and rendering the badge as `inherit` instead of `auto`. Reload of the default role also lost the auto state whenever the role value carried an explicit `:auto` suffix instead of relying on `defaultThinkingLevel`.

Widen the resolver chain (`parseThinkingSuffix`, `splitThinkingSuffix`, `parseModelString`, `parseModelPattern*`, `ResolvedModelRoleValue`, `ResolvedRoleModel`, `ResolveCliModelResult`) to carry the `AUTO_THINKING` sentinel end to end, and coerce it back to `undefined` at concrete-only boundaries (glob scope patterns, retry fallback, advisor, commit pipeline, guided-goal, bench).

Regression tests cover:

- `resolveModelRoleValue("provider/model:auto")` returns explicit auto without a warning.

- `ModelSelector` renders `DEFAULT (auto)` and `SMOL (auto)` when the role value has `:auto`.

- `cycleRoleModels` activates auto thinking on entering a `:auto` role.

- Startup resume activates auto thinking when `modelRoles.default` carries `:auto`.

Fixes #4128

packages/coding-agent/src/utils/commit-message-generator.ts

@@ -12,7 +12,7 @@ import { getModelMatchPreferences, resolveModelRoleValue } from "../config/model
 import type { Settings } from "../config/settings";
 import MODEL_PRIO from "../priority.json" with { type: "json" };
 import commitSystemPrompt from "../prompts/system/commit-message-system.md" with { type: "text" };
-import { toReasoningEffort } from "../thinking";
+import { AUTO_THINKING, toReasoningEffort } from "../thinking";
 
 const COMMIT_SYSTEM_PROMPT = prompt.render(commitSystemPrompt);
 const MAX_DIFF_CHARS = 4000;
@@ -56,7 +56,10 @@ function getSmolModelCandidates(
 		settings,
 		matchPreferences,
 	});
-	addCandidate(configuredSmol.model, configuredSmol.thinkingLevel);
+	addCandidate(
+		configuredSmol.model,
+		configuredSmol.thinkingLevel === AUTO_THINKING ? undefined : configuredSmol.thinkingLevel,
+	);
 
 	for (const pattern of MODEL_PRIO.smol) {
 		const needle = pattern.toLowerCase();