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

tests/test-stream.ts

@@ -302,6 +302,29 @@ describe("streamCommandCode — request serialization", () => {
     assert.equal(objectAt(server.lastRequestBody(), ["params", "max_tokens"]), 8_192)
   })
 
+  it("serializes OMP system prompt arrays as a string", async () => {
+    server.mockResponse({
+      type: "success",
+      events: [JSON.stringify({ type: "finish", finishReason: "stop" })],
+    })
+    const { streamCommandCode } = createTestDeps({ apiBase: server.baseUrl() })
+
+    await collectEvents(
+      streamCommandCode(
+        makeModel(),
+        makeContext({
+          systemPrompt: ["You are a test assistant.", "Use concise answers."] as unknown as string,
+        }),
+        { apiKey: "mock-key" },
+      ),
+    )
+
+    assert.equal(
+      objectAt(server.lastRequestBody(), ["params", "system"]),
+      "You are a test assistant.\n\nUse concise answers.",
+    )
+  })
+
   it("runs onPayload and onResponse hooks", async () => {
     server.mockResponse({
       type: "success",