fix: use exec.LookPath instead of 'command -v' for binary detection

'command' is a shell builtin, not a real binary, so exec.Command
always failed with exit 127. Replaced with os/exec.LookPath which
correctly resolves binaries in PATH.

Also adds capabilities_test.go to verify Probe() matches
exec.LookPath results.

Version: 0.7.2

Makefile

@@ -1,5 +1,5 @@
 BINARY=nasctl
-VERSION?=0.7.1
+VERSION?=0.7.2
 GO?=go
 LDFLAGS=-s -w -X github.com/darroyo/nasctl/internal/web.Version=$(VERSION) -X github.com/darroyo/nasctl/internal/web.Commit=$(shell git rev-parse --short HEAD 2>/dev/null || echo unknown)
 BUILD_FLAGS=CGO_ENABLED=0