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

Cargo.toml

@@ -9,3 +9,6 @@ signal-hook = "0.4.3"
 toml = "1.0.3"
 serde = { version = "1.0", features = ["derive"] }
 clap = { version = "4.5", features = ["derive"] }
+pulldown-cmark = "0.13.1"
+syntect = { version = "5.3", default-features = false, features = ["default-fancy"] }
+syntect-tui = "3.0"