feat(02-01): add Phase 2 deps and vault file-loading module

- Add pulldown-cmark 0.13.1, syntect 5.3 (default-fancy), syntect-tui 3.0 to Cargo.toml
- Create src/vault.rs with VaultDocument enum (Loaded/Missing/ReadError variants)
- Implement load_document() using std::fs::read_to_string with structured error mapping
- cargo check passes with all new dependencies resolved

src/vault.rs

@@ -0,0 +1,42 @@
+use std::io;
+use std::path::{Path, PathBuf};
+
+// ── VaultDocument ─────────────────────────────────────────────────────────────
+
+/// Represents the outcome of attempting to load a markdown file from the vault.
+#[allow(dead_code)]
+pub enum VaultDocument {
+    /// File was read successfully.
+    Loaded { path: PathBuf, content: String },
+    /// File does not exist at the resolved path.
+    Missing { path: PathBuf },
+    /// File exists or path was valid, but an I/O error occurred during reading.
+    ReadError { path: PathBuf, reason: String },
+}
+
+// ── load_document ─────────────────────────────────────────────────────────────
+
+/// Load a markdown document from the vault.
+///
+/// Joins `vault_path` with `relative` (e.g. `"index.md"`) to form the full
+/// file path, then reads the file to a String.
+///
+/// Returns:
+/// - `VaultDocument::Loaded`    — file read successfully
+/// - `VaultDocument::Missing`   — file does not exist
+/// - `VaultDocument::ReadError` — any other I/O error
+pub fn load_document(vault_path: &Path, relative: &str) -> VaultDocument {
+    let full_path = vault_path.join(relative);
+
+    match std::fs::read_to_string(&full_path) {
+        Ok(content) => VaultDocument::Loaded {
+            path: full_path,
+            content,
+        },
+        Err(e) if e.kind() == io::ErrorKind::NotFound => VaultDocument::Missing { path: full_path },
+        Err(e) => VaultDocument::ReadError {
+            path: full_path,
+            reason: e.to_string(),
+        },
+    }
+}