fix(mcp): fall back to lane cwd when source_repo is null

Adopted lanes have source_repo = null; lane-mcp.js passed it raw to
readSourceMcpServers and threw ENOMCPCONFIG. Match the existing fallback
pattern in lane-env.js/lane-profile.js.

server/__tests__/lane-mcp.test.js

@@ -79,6 +79,13 @@ describe("syncMcp — reading and relocating", () => {
       `${lane.cwd}/.playwright-mcp/profiles/default`
     );
   });
+
+  it("falls back to the lane's own cwd when source_repo is null (adopted lane)", async () => {
+    const lane = makeLane(null);
+    writeClaudeJson({ [lane.cwd]: { mcpServers: { playwright: { command: "npx", args: [] } } } });
+    const result = await laneMcp.syncMcp(lane);
+    assert.deepEqual(result.servers, ["playwright"]);
+  });
 });
 
 describe("syncMcp — Playwright output-dir pinning", () => {