feat: add pluggable multi-CLI backend system

Implement BackendAdapter interface with four CLI backends:
- ClaudeCodeBackend (extracted from AgentRuntime)
- CodexBackend (OpenAI Codex CLI)
- GeminiBackend (Google Gemini CLI)
- OpenCodeBackend (OpenCode CLI)

Add BackendRegistry for resolution/creation via AGENT_BACKEND env var.
Refactor AgentRuntime to delegate to BackendAdapter instead of
hardcoding Claude CLI. Update GatewayConfig with new env vars
(AGENT_BACKEND, BACKEND_CLI_PATH, BACKEND_MODEL, BACKEND_MAX_TURNS).

Includes 10 property-based test files and unit tests for edge cases.

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

@@ -0,0 +1,95 @@
+import { describe, it } from "vitest";
+import fc from "fast-check";
+import { CodexBackend } from "../../src/backends/codex-backend.js";
+import type { BackendAdapterConfig } from "../../src/backends/types.js";
+
+// Feature: multi-cli-backend, Property 2: Codex backend required flags
+// **Validates: Requirements 3.2, 3.3, 3.4, 3.5**
+
+/**
+ * Arbitrary for non-empty strings that won't break CLI arg parsing.
+ */
+const nonEmptyString = fc.string({ minLength: 1, maxLength: 200 });
+
+/**
+ * Arbitrary for working directory paths (non-empty, path-like).
+ */
+const workingDir = fc.stringMatching(/^\/[A-Za-z0-9_/.-]{1,100}$/);
+
+function createBackend(workDir: string): CodexBackend {
+  const config: BackendAdapterConfig = {
+    cliPath: "codex",
+    workingDir: workDir,
+    queryTimeoutMs: 60000,
+    allowedTools: [],
+    maxTurns: 25,
+  };
+  return new CodexBackend(config);
+}
+
+describe("Property 2: Codex backend required flags", () => {
+  it("generated args always contain the exec subcommand", () => {
+    fc.assert(
+      fc.property(
+        nonEmptyString,
+        workingDir,
+        (prompt, workDir) => {
+          const backend = createBackend(workDir);
+          const args = backend.buildArgs(prompt);
+
+          return args[0] === "exec";
+        },
+      ),
+      { numRuns: 100 },
+    );
+  });
+
+  it("generated args always contain --json", () => {
+    fc.assert(
+      fc.property(
+        nonEmptyString,
+        workingDir,
+        (prompt, workDir) => {
+          const backend = createBackend(workDir);
+          const args = backend.buildArgs(prompt);
+
+          return args.includes("--json");
+        },
+      ),
+      { numRuns: 100 },
+    );
+  });
+
+  it("generated args always contain --dangerously-bypass-approvals-and-sandbox", () => {
+    fc.assert(
+      fc.property(
+        nonEmptyString,
+        workingDir,
+        (prompt, workDir) => {
+          const backend = createBackend(workDir);
+          const args = backend.buildArgs(prompt);
+
+          return args.includes("--dangerously-bypass-approvals-and-sandbox");
+        },
+      ),
+      { numRuns: 100 },
+    );
+  });
+
+  it("generated args always contain --cd with the configured working directory", () => {
+    fc.assert(
+      fc.property(
+        nonEmptyString,
+        workingDir,
+        (prompt, workDir) => {
+          const backend = createBackend(workDir);
+          const args = backend.buildArgs(prompt);
+
+          const cdIndex = args.indexOf("--cd");
+          return cdIndex !== -1 && args[cdIndex + 1] === workDir;
+        },
+      ),
+      { numRuns: 100 },
+    );
+  });
+});