Stamp the git tag or short commit into the binary (closes #10)

rtnetmon had no version. main.Version is now set at link time and logged
in the first startup line. `make build` and `make dev` set it from
`git describe --tags --always` unless VERSION is given. The Dockerfile
takes it from the VERSION build argument when one is given, otherwise
from `git describe --tags --always` of the .git the build context now
carries, and fails the build if the context carries .git and no version
comes out. .dockerignore follows the canonical copy: .git is sent,
.git/config, which can hold a credential, is not.

Model: opus-5-5

internal/cli/export_test.go

@@ -3,4 +3,4 @@ package cli
 import "github.com/spf13/cobra"
 
 // NewRootCmd exposes newRootCmd for external tests.
-func NewRootCmd() *cobra.Command { return newRootCmd() }
+func NewRootCmd() *cobra.Command { return newRootCmd("dev") }

internal/cli/root.go

@@ -51,8 +51,9 @@ func defaultTCPHosts() []string {
 	}
 }
 
-// newRootCmd builds the cobra root command with its flags bound.
-func newRootCmd() *cobra.Command {
+// newRootCmd builds the cobra root command with its flags bound. version is
+// logged at startup.
+func newRootCmd(version string) *cobra.Command {
 	cfg := &config{}
 	cmd := &cobra.Command{
 		Use:   "rtnetmon",
@@ -60,7 +61,7 @@ func newRootCmd() *cobra.Command {
 		Long: `rtnetmon is a dual-interface network monitoring dashboard that provides
 real-time visibility into network health, packet loss, and latency.`,
 		RunE: func(cmd *cobra.Command, _ []string) error {
-			return runMonitor(cmd, cfg)
+			return runMonitor(cmd, cfg, version)
 		},
 	}
 	registerFlags(cmd, cfg)
@@ -88,8 +89,8 @@ func registerFlags(cmd *cobra.Command, cfg *config) {
 
 // runMonitor detects the interfaces to monitor, then constructs and runs the
 // monitor from cfg.
-func runMonitor(cmd *cobra.Command, cfg *config) error {
-	monitor.Logf(cfg.LogFile, "Starting rtnetmon")
+func runMonitor(cmd *cobra.Command, cfg *config, version string) error {
+	monitor.Logf(cfg.LogFile, "Starting rtnetmon %s", version)
 
 	specs, err := detectInterfaces(cmd, cfg)
 	if err != nil {
@@ -169,7 +170,7 @@ func detectInterfaces(
 	return specs, nil
 }
 
-// Execute builds the root command and runs it.
-func Execute() error {
-	return newRootCmd().Execute()
+// Execute builds the root command and runs it. version is logged at startup.
+func Execute(version string) error {
+	return newRootCmd(version).Execute()
 }