Adds sim/hub_leaf_test.go: a real *hub.Hub bound to auto-allocated ports, a real *agent.Agent built against a leaf repo cloned from the hub, and a round-trip Commit + SyncTo + Hub.ReadAt that exercises the HTTP-xfer path end-to-end.

Without this test, the kind of cross-component regression bones surfaced in #120 (the SetMaxOpenConns deadlock under concurrent clones) only shows up downstream. With it, EdgeSync's own CI catches it.

Wired into the existing 'Sim serve tests' step (Makefile + test.yml) alongside TestServeHTTP/TestLeafToLeaf/TestAgentServe so the test runs on every PR.

Closes #113 leaf check-in: 7275783d59 user: [email protected] tags: trunk, feat/113-hub-leaf-e2e