Migrate devcontainer to custom Dockerfile

Move configuration to .devcontainer directory, update to OpenJDK 25, and
add essential tooling like hyperfine and lazygit.

.devcontainer.json

@@ -1,18 +0,0 @@
-{
-    "name": "Java",
-    "image": "mcr.microsoft.com/vscode/devcontainers/base:1-ubuntu-24.04",
-    "features": {
-	"ghcr.io/guiyomh/features/just:0": {},
-        "ghcr.io/devcontainers/features/java:1": {
-            "version": "24-open",
-            "installMaven": true
-        }
-    },
-    "customizations": {
-        "vscode": {
-            "extensions": [
-                "vscjava.vscode-java-pack"
-            ]
-        }
-    }
-}

.devcontainer/Dockerfile

@@ -0,0 +1,3 @@
+# Debian 13 (Trixie) + OpenJDK 25
+FROM mcr.microsoft.com/devcontainers/java:25-trixie
+RUN apt-get update && apt-get install -y maven just hyperfine lazygit

.devcontainer/devcontainer.json

@@ -0,0 +1,14 @@
+{
+    "name": "Java",
+    "build": {
+        "dockerfile": "Dockerfile"
+    },
+    "customizations": {
+        "vscode": {
+            "extensions": ["vscjava.vscode-java-pack"]
+        },
+        "zed": {
+            "extensions": ["java", "just", "xml"]
+        }
+    }
+}