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/opencode-backend.property.test.ts

@@ -0,0 +1,94 @@
+import { describe, it } from "vitest";
+import fc from "fast-check";
+import { OpenCodeBackend } from "../../src/backends/opencode-backend.js";
+import type { BackendAdapterConfig } from "../../src/backends/types.js";
+
+// Feature: multi-cli-backend, Property 4: OpenCode backend required flags
+// **Validates: Requirements 5.2, 5.3, 5.5**
+
+/**
+ * Arbitrary for non-empty strings that won't break CLI arg parsing.
+ */
+const nonEmptyString = fc.string({ minLength: 1, maxLength: 200 });
+
+/**
+ * Arbitrary for model strings (provider/model format).
+ */
+const modelString = fc.stringMatching(/^[a-z]{1,20}\/[a-z0-9-]{1,40}$/);
+
+function createBackend(model?: string): OpenCodeBackend {
+  const config: BackendAdapterConfig = {
+    cliPath: "opencode",
+    workingDir: "/tmp",
+    queryTimeoutMs: 60000,
+    allowedTools: [],
+    maxTurns: 25,
+    model,
+  };
+  return new OpenCodeBackend(config);
+}
+
+describe("Property 4: OpenCode backend required flags", () => {
+  it("generated args always start with the run subcommand", () => {
+    fc.assert(
+      fc.property(
+        nonEmptyString,
+        (prompt) => {
+          const backend = createBackend();
+          const args = backend.buildArgs(prompt);
+
+          return args[0] === "run";
+        },
+      ),
+      { numRuns: 100 },
+    );
+  });
+
+  it("generated args always contain --format json", () => {
+    fc.assert(
+      fc.property(
+        nonEmptyString,
+        (prompt) => {
+          const backend = createBackend();
+          const args = backend.buildArgs(prompt);
+
+          const formatIndex = args.indexOf("--format");
+          return formatIndex !== -1 && args[formatIndex + 1] === "json";
+        },
+      ),
+      { numRuns: 100 },
+    );
+  });
+
+  it("generated args contain --model when a model is configured", () => {
+    fc.assert(
+      fc.property(
+        nonEmptyString,
+        modelString,
+        (prompt, model) => {
+          const backend = createBackend(model);
+          const args = backend.buildArgs(prompt);
+
+          const modelIndex = args.indexOf("--model");
+          return modelIndex !== -1 && args[modelIndex + 1] === model;
+        },
+      ),
+      { numRuns: 100 },
+    );
+  });
+
+  it("generated args do not contain --model when no model is configured", () => {
+    fc.assert(
+      fc.property(
+        nonEmptyString,
+        (prompt) => {
+          const backend = createBackend(undefined);
+          const args = backend.buildArgs(prompt);
+
+          return !args.includes("--model");
+        },
+      ),
+      { numRuns: 100 },
+    );
+  });
+});