feat(notsecrets): X25519 identity and recipient with wrap/unwrap

Implements ECDH key wrapping via HKDF-SHA256 + ChaCha20Poly1305,
bech32 encode/decode for age1... recipients and AGE-SECRET-KEY-1... identities.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/notsecrets/src/identities/mod.rs

@@ -1,5 +1,8 @@
 use crate::error::AgeError;
 
+pub mod x25519;
+pub use x25519::X25519Identity;
+
 /// The symmetric file encryption key — 16 random bytes.
 #[derive(Clone)]
 pub struct FileKey([u8; 16]);