chore: remove dead code, bump to 0.7.2

- Remove unused delete_master_key from crypto.rs
- Remove unused audit::log from audit.rs
- Simplify HistoryRow in rollback.rs (drop unused namespace/kind/name)
- Update AGENTS.md: audit::log → audit::log_tx

Made-with: Cursor

src/crypto.rs

@@ -105,15 +105,6 @@ pub fn store_master_key(key: &[u8; 32]) -> Result<()> {
     Ok(())
 }
 
-/// Delete the Master Key from the OS Keychain (used by tests / reset).
-#[cfg(test)]
-pub fn delete_master_key() -> Result<()> {
-    let entry =
-        keyring::Entry::new(KEYRING_SERVICE, KEYRING_USER).context("create keychain entry")?;
-    let _ = entry.delete_credential();
-    Ok(())
-}
-
 // ─── Minimal hex helpers (avoid extra dep) ────────────────────────────────────
 
 mod hex {