Add subdirectory file discovery to arc-devcontainer

When neither .devcontainer/devcontainer.json nor .devcontainer.json
exists, scan .devcontainer/ for subdirectories containing a
devcontainer.json. Subdirectories are sorted alphabetically and the
first match is used. Standard locations still take priority.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/arc-devcontainer/src/lib.rs

@@ -357,17 +357,57 @@ impl DevcontainerResolver {
             return Ok((path.to_path_buf(), parsed));
         }
 
+        // Subdirectory format: scan .devcontainer/ for subdirs containing devcontainer.json
+        let devcontainer_dir = path.join(".devcontainer");
+        if devcontainer_dir.is_dir() {
+            let mut subdirs: Vec<PathBuf> = std::fs::read_dir(&devcontainer_dir)
+                .map_err(|source| DevcontainerError::ReadFile {
+                    path: devcontainer_dir.clone(),
+                    source,
+                })?
+                .filter_map(|entry| entry.ok())
+                .filter(|entry| entry.path().is_dir())
+                .map(|entry| entry.path())
+                .filter(|dir| dir.join("devcontainer.json").exists())
+                .collect();
+
+            // Sort alphabetically to get deterministic first pick
+            subdirs.sort();
+
+            if let Some(subdir) = subdirs.first() {
+                let candidate = subdir.join("devcontainer.json");
+                let raw = std::fs::read_to_string(&candidate).map_err(|source| {
+                    DevcontainerError::ReadFile {
+                        path: candidate.clone(),
+                        source,
+                    }
+                })?;
+                let stripped = jsonc::strip_jsonc(&raw);
+                let parsed: DevcontainerJson = serde_json::from_str(&stripped)?;
+                return Ok((candidate, parsed));
+            }
+        }
+
         Err(DevcontainerError::NotFound(path.to_path_buf()))
     }
 
     fn repo_root_from_json_path<'a>(json_path: &Path, original_path: &'a Path) -> &'a Path {
+        // If json_path is inside .devcontainer/<subdir>/, the repo root is two levels up
         // If json_path is inside .devcontainer/, the repo root is one level up
         if let Some(parent) = json_path.parent() {
             if parent.file_name().is_some_and(|n| n == ".devcontainer") {
                 if let Some(repo_root) = parent.parent() {
-                    // Only return repo_root if it matches the original path structure
                     let _ = repo_root;
                 }
+            } else if let Some(grandparent) = parent.parent() {
+                if grandparent
+                    .file_name()
+                    .is_some_and(|n| n == ".devcontainer")
+                {
+                    if let Some(repo_root) = grandparent.parent() {
+                        let _ = repo_root;
+                    }
+                }
             }
         }
         original_path

crates/arc-devcontainer/tests/fixtures/subdirectory-mode/.devcontainer/python/devcontainer.json

@@ -0,0 +1,4 @@
+{
+  "image": "mcr.microsoft.com/devcontainers/python:3.12",
+  "remoteUser": "vscode"
+}

crates/arc-devcontainer/tests/fixtures/subdirectory-multiple/.devcontainer/alpha/devcontainer.json

@@ -0,0 +1,4 @@
+{
+  "image": "mcr.microsoft.com/devcontainers/base:ubuntu",
+  "remoteUser": "alpha-user"
+}

crates/arc-devcontainer/tests/fixtures/subdirectory-multiple/.devcontainer/beta/devcontainer.json

@@ -0,0 +1,4 @@
+{
+  "image": "node:20",
+  "remoteUser": "beta-user"
+}

crates/arc-devcontainer/tests/fixtures/subdirectory-with-standard/.devcontainer/devcontainer.json

@@ -0,0 +1,4 @@
+{
+  "image": "mcr.microsoft.com/devcontainers/base:ubuntu",
+  "remoteUser": "standard-user"
+}

crates/arc-devcontainer/tests/fixtures/subdirectory-with-standard/.devcontainer/python/devcontainer.json

@@ -0,0 +1,4 @@
+{
+  "image": "mcr.microsoft.com/devcontainers/python:3.12",
+  "remoteUser": "python-user"
+}

crates/arc-devcontainer/tests/integration.rs

@@ -126,6 +126,39 @@ async fn resolve_not_found() {
     assert!(err.to_string().contains("no devcontainer.json found"));
 }
 
+#[tokio::test]
+async fn resolve_subdirectory_mode() {
+    let config = DevcontainerResolver::resolve(&fixture_path("subdirectory-mode"))
+        .await
+        .unwrap();
+
+    assert!(config.dockerfile.contains("FROM mcr.microsoft.com/devcontainers/python:3.12"));
+    assert_eq!(config.remote_user.as_deref(), Some("vscode"));
+    assert_eq!(config.workspace_folder, "/workspaces/subdirectory-mode");
+}
+
+#[tokio::test]
+async fn resolve_subdirectory_multiple_picks_alphabetical_first() {
+    let config = DevcontainerResolver::resolve(&fixture_path("subdirectory-multiple"))
+        .await
+        .unwrap();
+
+    // "alpha" sorts before "beta", so alpha's config is used
+    assert!(config.dockerfile.contains("FROM mcr.microsoft.com/devcontainers/base:ubuntu"));
+    assert_eq!(config.remote_user.as_deref(), Some("alpha-user"));
+}
+
+#[tokio::test]
+async fn resolve_subdirectory_standard_wins_over_subdirs() {
+    let config = DevcontainerResolver::resolve(&fixture_path("subdirectory-with-standard"))
+        .await
+        .unwrap();
+
+    // Standard .devcontainer/devcontainer.json takes priority over subdirectory format
+    assert!(config.dockerfile.contains("FROM mcr.microsoft.com/devcontainers/base:ubuntu"));
+    assert_eq!(config.remote_user.as_deref(), Some("standard-user"));
+}
+
 #[tokio::test]
 async fn generated_dockerfile_is_well_formed() {
     let config = DevcontainerResolver::resolve(&fixture_path("image-only"))