fix(milestone): add missing cmd/orca/main.go entry point

The P01 ship commit (503923b) merged docs and metadata for the CLI
skeleton phase but the cmd/orca/main.go entry point was never committed
to the milestone branch. It existed only as a working-tree file.

This commit adds the 15-line entry point that wires cli.Execute() into
a runnable orca binary. The unbuilt file was shadowing the build via
go's automatic working-tree pickup.

---ci---
project: orca
phase: 0
milestone: v0.1
status: fix
---/ci---

cmd/orca/main.go

@@ -0,0 +1,15 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"git.cloudinit.dev/coreci/orca/internal/cli"
+)
+
+func main() {
+	if err := cli.Execute(); err != nil {
+		fmt.Fprintf(os.Stderr, "error: %v\n", err)
+		os.Exit(1)
+	}
+}