Support $PORT env var for MCP agent port allocation

MCP agent sets $PORT for containers with route declarations.
$PORT takes precedence over HTTP_PORT from config, matching the mcdsl
convention used by all other platform services.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

main.go

@@ -6,6 +6,7 @@ import (
 	"log"
 	"net"
 	"net/http"
+	"os"
 
 	"git.wntrmute.dev/kyle/goutils/config"
 	"git.wntrmute.dev/kyle/goutils/die"
@@ -29,10 +30,13 @@ func main() {
 	srv := &server{db: db}
 	http.Handle("/", srv)
 
-	addr := net.JoinHostPort(
-		config.Get("HTTP_ADDR"),
-		config.GetDefault("HTTP_PORT", "8000"),
-	)
+	// $PORT is set by the MCP agent for containers with route declarations.
+	// It takes precedence over config, matching the mcdsl convention.
+	port := os.Getenv("PORT")
+	if port == "" {
+		port = config.GetDefault("HTTP_PORT", "8000")
+	}
+	addr := net.JoinHostPort(config.Get("HTTP_ADDR"), port)
 	log.Print("listening on ", addr)
 	log.Fatal(http.ListenAndServe(addr, nil))
 }