feat(KB-137): add automatic port fallback when dashboard port is in use

- Handle EADDRINUSE by retrying with port 0 (OS-assigned random port)
- Print warning message showing original and fallback port
- Update banner and browser-open to use the actual bound port
- Add tests for successful listen, fallback, and warning behavior
- Add changeset for patch release

.changeset/auto-port-fallback.md

@@ -0,0 +1,5 @@
+---
+"@dustinbyrne/kb": patch
+---
+
+Auto-assign random port when dashboard port is already in use instead of crashing with EADDRINUSE.