feat: add OMP (Oh My Pi) provider compatibility

Support OMP as a host alongside pi:
- Convert OMP's array-format system prompts to string via systemPromptToText
- Guard against OMP passing the literal env-var name as the API key value
- Add ~/.omp/agent/auth.json to default auth path lookup
- Fix --list-models output to check both stdout and stderr
- Add OMP compatibility smoke test with isolated temp HOME and mock server
- Deduplicate usage section in README, add OMP install and usage docs

src/converters.ts

@@ -39,7 +39,7 @@ export function numberValue(value: unknown): number | undefined {
 }
 
 function defaultAuthPaths(home: string): string[] {
-  return [join(home, ".commandcode", "auth.json"), join(home, ".pi", "agent", "auth.json")]
+  return [join(home, ".commandcode", "auth.json"), join(home, ".omp", "agent", "auth.json"), join(home, ".pi", "agent", "auth.json")]
 }
 
 function apiKeyFromCredentialRecord(value: unknown): string | undefined {
@@ -284,3 +284,23 @@ export function mapFinishReason(reason: unknown): StopReason {
   }
   return "stop"
 }
+
+function promptPartToText(value: unknown, depth = 0): string {
+  if (depth > 10) return ""
+  if (typeof value === "string") return value
+  if (Array.isArray(value)) return value.map((v) => promptPartToText(v, depth + 1)).filter(Boolean).join("\n")
+  if (!isRecord(value)) return ""
+  const text = stringValue(value.text)
+  if (text) return text
+  const content = promptPartToText(value.content, depth + 1)
+  if (content) return content
+  return ""
+}
+
+export function systemPromptToText(value: unknown): string {
+  if (value === undefined || value === null) return ""
+  if (typeof value === "string") return value
+  if (Array.isArray(value)) return value.map((v) => promptPartToText(v, 0)).filter(Boolean).join("\n\n")
+  return promptPartToText(value, 0)
+}
+