feat(04-02): add [[Directory]] sentinel and list_vault_files() to vault.rs

- resolve_wiki_link() returns PathBuf::from("__directory__") for case-insensitive 'directory' target
- DirEntry struct with depth, name, is_dir, vault_path fields
- list_vault_files() using WalkDir::new().sort_by_file_name() for alphabetical tree
- Skips hidden entries (leading '.') and non-.md files silently

src/vault.rs

@@ -1,5 +1,6 @@
 use std::io;
 use std::path::{Path, PathBuf};
+use walkdir::WalkDir;
 
 // ── VaultDocument ─────────────────────────────────────────────────────────────
 
@@ -83,6 +84,11 @@ pub fn is_within_vault(vault_path: &Path, candidate: &Path) -> bool {
 ///   → matches "guides/getting-started.md"
 /// ```
 pub fn resolve_wiki_link(vault_path: &Path, raw_target: &str) -> Option<PathBuf> {
+    // Magic sentinel — [[Directory]] navigates to the virtual directory page
+    if raw_target.eq_ignore_ascii_case("directory") {
+        return Some(PathBuf::from("__directory__"));
+    }
+
     // Split off subpath prefix if present: "guides/Getting Started" → ("guides", "Getting Started")
     let (subdir, name) = match raw_target.rfind('/') {
         Some(i) => (&raw_target[..i], &raw_target[i + 1..]),
@@ -182,3 +188,66 @@ pub fn resolve_standard_link(vault_path: &Path, current_doc: &str, dest: &str) -
 
     None
 }
+
+// ── Directory listing ─────────────────────────────────────────────────────────
+
+/// Entry in the vault directory listing.
+pub struct DirEntry {
+    /// Indentation depth (1-based, depth 1 = vault root children).
+    pub depth: usize,
+    /// Display name (without .md extension for files).
+    pub name: String,
+    /// True if this entry is a directory.
+    pub is_dir: bool,
+    /// Vault-relative path (only for .md files; None for directories).
+    pub vault_path: Option<String>,
+}
+
+/// List all markdown files and directories in the vault, sorted alphabetically.
+///
+/// Skips hidden files/directories (starting with '.') and non-.md files.
+/// Returns entries with depth for tree indentation.
+pub fn list_vault_files(vault_path: &Path) -> Vec<DirEntry> {
+    let mut entries = Vec::new();
+    for entry in WalkDir::new(vault_path)
+        .sort_by_file_name()
+        .into_iter()
+        .filter_map(|e| e.ok())
+        .skip(1) // skip vault root itself
+    {
+        let depth = entry.depth();
+        let name = entry.file_name().to_string_lossy().to_string();
+
+        // Skip hidden files/dirs
+        if name.starts_with('.') {
+            continue;
+        }
+
+        let is_dir = entry.file_type().is_dir();
+
+        if is_dir {
+            entries.push(DirEntry {
+                depth,
+                name,
+                is_dir: true,
+                vault_path: None,
+            });
+        } else if name.ends_with(".md") {
+            let display_name = name.strip_suffix(".md").unwrap_or(&name).to_string();
+            let rel = entry
+                .path()
+                .strip_prefix(vault_path)
+                .unwrap_or(entry.path())
+                .to_string_lossy()
+                .to_string();
+            entries.push(DirEntry {
+                depth,
+                name: display_name,
+                is_dir: false,
+                vault_path: Some(rel),
+            });
+        }
+        // Skip non-.md files silently
+    }
+    entries
+}