fix: root-anchor mcables ignore pattern, commit cmd/mcables/main.go

The bare `mcables` pattern in .gitignore (line 11) and .dockerignore
(line 18) was intended to ignore the built binary at the repo root, but
without a leading slash it also matched the cmd/mcables/ directory. The
result: cmd/mcables/main.go was never tracked in git, and fresh worktrees
had to copy it from a sibling to build.

- Change `mcables` → `/mcables` in both files (still ignores the root
  binary; no longer matches the cmd subdirectory).
- Add cmd/mcables/main.go (copied from picasso's worktree, verified
  identical to head's main checkout).

Verified: `git check-ignore cmd/mcables/main.go` returns not-ignored;
a touched `./mcables` at the repo root is still ignored via `/mcables`.
`go build ./...` clean.

.dockerignore

@@ -15,7 +15,7 @@ data
 
 # Build artefacts
 bin
-mcables
+/mcables
 
 # Editor cruft
 .vscode