feat(cli): add UnlockedVault session wrapping master_key in Zeroizing

Provides load/save helpers for Manifest/Settings/Item; atomic_write keeps
vault files consistent across crashes. main.rs is transiently broken
against the old Entry API — Task 5+ rewrites the command handlers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/relicario-cli/src/main.rs

@@ -38,6 +38,7 @@
 //! creates a git commit, preserving an audit log of all vault changes.
 
 mod helpers;
+mod session;
 
 use anyhow::{bail, Context, Result};
 use clap::{Parser, Subcommand};