feat: support local plugin marketplace paths

Enable installing plugins from local directories in addition to remote
Git URLs. This allows users to use local plugin marketplaces within their
repository without requiring them to be hosted in a separate Git repo.

Example usage:
  plugin_marketplaces: "./my-local-marketplace"
  plugins: "my-plugin@my-local-marketplace"

Supported path formats:
- Relative paths: ./plugins, ../shared-plugins
- Absolute Unix paths: /home/user/plugins
- Absolute Windows paths: C:\Users\user\plugins

Fixes #664

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

base-action/test/install-plugins.test.ts

@@ -596,4 +596,111 @@ describe("installPlugins", () => {
       { stdio: "inherit" },
     );
   });
+
+  // Local marketplace path tests
+  test("should accept local marketplace path with ./", async () => {
+    const spy = createMockSpawn();
+    await installPlugins("./my-local-marketplace", "test-plugin");
+
+    expect(spy).toHaveBeenCalledTimes(2);
+    expect(spy).toHaveBeenNthCalledWith(
+      1,
+      "claude",
+      ["plugin", "marketplace", "add", "./my-local-marketplace"],
+      { stdio: "inherit" },
+    );
+    expect(spy).toHaveBeenNthCalledWith(
+      2,
+      "claude",
+      ["plugin", "install", "test-plugin"],
+      { stdio: "inherit" },
+    );
+  });
+
+  test("should accept local marketplace path with absolute Unix path", async () => {
+    const spy = createMockSpawn();
+    await installPlugins("/home/user/my-marketplace", "test-plugin");
+
+    expect(spy).toHaveBeenCalledTimes(2);
+    expect(spy).toHaveBeenNthCalledWith(
+      1,
+      "claude",
+      ["plugin", "marketplace", "add", "/home/user/my-marketplace"],
+      { stdio: "inherit" },
+    );
+  });
+
+  test("should accept local marketplace path with Windows absolute path", async () => {
+    const spy = createMockSpawn();
+    await installPlugins("C:\\Users\\user\\marketplace", "test-plugin");
+
+    expect(spy).toHaveBeenCalledTimes(2);
+    expect(spy).toHaveBeenNthCalledWith(
+      1,
+      "claude",
+      ["plugin", "marketplace", "add", "C:\\Users\\user\\marketplace"],
+      { stdio: "inherit" },
+    );
+  });
+
+  test("should accept mixed local and remote marketplaces", async () => {
+    const spy = createMockSpawn();
+    await installPlugins(
+      "./local-marketplace\nhttps://github.com/user/remote.git",
+      "test-plugin",
+    );
+
+    expect(spy).toHaveBeenCalledTimes(3);
+    expect(spy).toHaveBeenNthCalledWith(
+      1,
+      "claude",
+      ["plugin", "marketplace", "add", "./local-marketplace"],
+      { stdio: "inherit" },
+    );
+    expect(spy).toHaveBeenNthCalledWith(
+      2,
+      "claude",
+      ["plugin", "marketplace", "add", "https://github.com/user/remote.git"],
+      { stdio: "inherit" },
+    );
+  });
+
+  test("should accept local path with ../ (parent directory)", async () => {
+    const spy = createMockSpawn();
+    await installPlugins("../shared-plugins/marketplace", "test-plugin");
+
+    expect(spy).toHaveBeenCalledTimes(2);
+    expect(spy).toHaveBeenNthCalledWith(
+      1,
+      "claude",
+      ["plugin", "marketplace", "add", "../shared-plugins/marketplace"],
+      { stdio: "inherit" },
+    );
+  });
+
+  test("should accept local path with nested directories", async () => {
+    const spy = createMockSpawn();
+    await installPlugins("./plugins/my-org/my-marketplace", "test-plugin");
+
+    expect(spy).toHaveBeenCalledTimes(2);
+    expect(spy).toHaveBeenNthCalledWith(
+      1,
+      "claude",
+      ["plugin", "marketplace", "add", "./plugins/my-org/my-marketplace"],
+      { stdio: "inherit" },
+    );
+  });
+
+  test("should accept local path with dots in directory name", async () => {
+    const spy = createMockSpawn();
+    await installPlugins("./my.plugin.marketplace", "test-plugin");
+
+    expect(spy).toHaveBeenCalledTimes(2);
+    expect(spy).toHaveBeenNthCalledWith(
+      1,
+      "claude",
+      ["plugin", "marketplace", "add", "./my.plugin.marketplace"],
+      { stdio: "inherit" },
+    );
+  });
 });