fix: hide commit and build time when unavailable

Only display commit hash and build time in --version output when they
have actual values. When using 'go install', these show as 'none' and
'unknown' which clutters the output.

Before (go install):
  plum version v0.3.5
    commit: none
    built: unknown

After (go install):
  plum version v0.3.5

After (Homebrew/binaries):
  plum version v0.3.5
    commit: 3b0135b
    built: 2026-01-11T07:51:08Z

Author: itsdevcoffee

cmd/plum/main.go

@@ -54,8 +54,14 @@ func main() {
 	if len(os.Args) > 1 && (os.Args[1] == "--version" || os.Args[1] == "-v") {
 		ver, cmt, bDate := getVersion()
 		fmt.Printf("plum version %s\n", ver)
-		fmt.Printf("  commit: %s\n", cmt)
-		fmt.Printf("  built: %s\n", bDate)
+
+		// Only show commit and build time if available
+		if cmt != "none" && cmt != "" {
+			fmt.Printf("  commit: %s\n", cmt)
+		}
+		if bDate != "unknown" && bDate != "" {
+			fmt.Printf("  built: %s\n", bDate)
+		}
 		os.Exit(0)
 	}