feat: layer runtime and model selection across conversation, project and global

- Store runtime selection as { provider?, model? } layers and resolve them per
  field (conversation -> project -> global); drop the legacy auto provider.
- Migrate stored selections (database v49) and clear deleted profile ids.
- Re-resolve selections in Main per request and at queue dispatch.
- Add ProjectRuntimeSelector and a two-column RuntimeModelPicker; manage
  link opens model connections; restore per-runtime default model tabs.
- Stop duplicating trusted instructions and history across OpenCode,
  Continue and DeepSeek Harness runtimes.
- Document the layered design, global provider removal plan and planned
  purpose-specific models.

src/main/agent/opencode-runtime.test.ts

@@ -207,6 +207,10 @@ function runClient(events: Record<string, unknown>[]) {
         data: true,
         error: undefined,
       }),
+      deleteMessage: vi.fn().mockResolvedValue({
+        data: true,
+        error: undefined,
+      }),
     },
     event: {
       subscribe: vi.fn().mockImplementation(async () => {
@@ -4155,6 +4159,35 @@ describe("OpenCodeRuntime embedded permission mediation", () => {
     await runtime.dispose();
   });
 
+  it("removes an unanswered prompt from a reused session after failure", async () => {
+    const { client, session } = runClient([
+      {
+        id: "event-error",
+        type: "session.error",
+        properties: {
+          sessionID: "session-1",
+          error: { name: "UnknownError", data: { message: "model failed" } },
+        },
+      },
+    ]);
+    const runtime = embeddedRuntime(client);
+
+    await expect(collectRun(runtime)).rejects.toThrow("model failed");
+
+    const promptMessageId = vi.mocked(session.promptAsync).mock.calls[0]?.[0]
+      ?.messageID;
+    expect(promptMessageId).toMatch(/^msg_/);
+    expect(session.deleteMessage).toHaveBeenCalledWith(
+      {
+        sessionID: "session-1",
+        messageID: promptMessageId,
+        directory: process.cwd(),
+      },
+      { signal: expect.any(AbortSignal) },
+    );
+    await runtime.dispose();
+  });
+
   it("deletes an ephemeral OpenCode session when released", async () => {
     const { client, session } = runClient([
       {