chore(gitignore): ignore only the root build binary, not the cmd/myday source

The bare `myday` pattern matched any path component named `myday`, including
the `cmd/myday/` source directory — so `cmd/myday/main.go` (the entrypoint)
could never be committed and a fresh clone would not build. Anchor it to the
repo root (`/myday`); the built binary still falls out of git.

.gitignore

@@ -19,7 +19,7 @@ data/
 
 # --- Go build output ---
 /bin/
-myday
+/myday
 *.exe
 
 # --- editor / OS ---