feat(manager): accept identity strings

[jgeluk]

What changed

• add SecretManager::from_identity_string
• keep parsing of age X25519 identities inside dotenvage
• test valid round-tripping and invalid identity rejection

Why

Consumers previously had to parse strings into age::x25519::Identity before constructing a SecretManager. That leaked dotenvage's concrete age dependency into consumers and required an exact matching age version.

Validation

• cargo test identity_string_constructor
• commit hook: cargo +nightly fmt -- --check
• commit hook: cargo +nightly clippy --all-targets -- -D warnings