feat: add Pi Coding Agent backend runtime, update dashboard UI

tests/property/cross-backend.property.test.ts

@@ -4,6 +4,7 @@ import { ClaudeCodeBackend } from "../../src/backends/claude-backend.js";
 import { CodexBackend } from "../../src/backends/codex-backend.js";
 import { GeminiBackend } from "../../src/backends/gemini-backend.js";
 import { OpenCodeBackend } from "../../src/backends/opencode-backend.js";
+import { PiBackend } from "../../src/backends/pi-backend.js";
 import type { BackendAdapterConfig } from "../../src/backends/types.js";
 
 // ── Shared arbitraries ──────────────────────────────────────────────
@@ -151,6 +152,35 @@ describe("Property 5: Session resume args across backends", () => {
       );
     });
   });
+
+  describe("Pi: --session <id> --continue when session provided, absent otherwise", () => {
+    it("includes --session <id> --continue when session ID is provided", () => {
+      fc.assert(
+        fc.property(nonEmptyString, nonEmptyString, sessionId, (prompt, sysPr, sid) => {
+          const backend = new PiBackend(makeConfig());
+          const args = backend.buildArgs(prompt, sysPr, sid);
+          const sessionIdx = args.indexOf("--session");
+          return (
+            sessionIdx !== -1 &&
+            args[sessionIdx + 1] === sid &&
+            args.includes("--continue")
+          );
+        }),
+        { numRuns: 100 },
+      );
+    });
+
+    it("does not include --session or --continue when no session ID is provided", () => {
+      fc.assert(
+        fc.property(nonEmptyString, nonEmptyString, (prompt, sysPr) => {
+          const backend = new PiBackend(makeConfig());
+          const args = backend.buildArgs(prompt, sysPr);
+          return !args.includes("--session") && !args.includes("--continue");
+        }),
+        { numRuns: 100 },
+      );
+    });
+  });
 });
 
 // ═══════════════════════════════════════════════════════════════════
@@ -229,6 +259,24 @@ describe("Property 6: Output parsing extracts correct fields", () => {
       { numRuns: 100 },
     );
   });
+
+  it("Pi: parses NDJSON with result type and session_id", () => {
+    fc.assert(
+      fc.property(responseText, sessionId, (text, sid) => {
+        const backend = new PiBackend(makeConfig());
+        const lines = [
+          JSON.stringify({ type: "result", result: text, session_id: sid }),
+        ].join("\n");
+        const result = backend.parseOutput(lines);
+        return (
+          result.isError === false &&
+          result.responseText === text &&
+          result.sessionId === sid
+        );
+      }),
+      { numRuns: 100 },
+    );
+  });
 });
 
 // ═══════════════════════════════════════════════════════════════════
@@ -252,6 +300,7 @@ const backendErrorPrefixes: Record<string, string> = {
   codex: "Codex CLI error",
   gemini: "Gemini CLI error",
   opencode: "OpenCode CLI error",
+  pi: "Pi CLI error",
 };
 
 /**
@@ -272,7 +321,7 @@ function simulateErrorResult(
 }
 
 describe("Property 8: Non-zero exit code produces error result", () => {
-  const backendNames = ["claude", "codex", "gemini", "opencode"] as const;
+  const backendNames = ["claude", "codex", "gemini", "opencode", "pi"] as const;
 
   it("for any backend, non-zero exit code and stderr, result has isError=true and responseText contains stderr", () => {
     fc.assert(