refactor: formalize FileStore and IdentitySource ports in notfiles

- Create FileStore trait in crates/notfiles/src/ports.rs capturing all file I/O
  operations (read, write, rename, remove, metadata, symlink, etc.)
- Create FileStoreImpl adapter in crates/notfiles/src/adapters/fs.rs wrapping std::fs
- Update linker.rs State::load/save and link_package/unlink_package to accept
  FileStore parameter for dependency injection
- Create IdentitySource port in crates/notsecrets/src/ports.rs
- Move IdentitySource trait from sources/mod.rs to ports.rs, maintain backward
  compatibility via re-export
- Update all source implementations to import from ports
- Update lib.rs in both crates to re-export ports
- Pass FileStoreImpl through call chain in main.rs and lib.rs public API

Maintains 100% backward compatible public API; FileStore dependency is internal.

crates/notsecrets/src/sources/mod.rs

@@ -1,17 +1,8 @@
-use crate::error::AgeError;
-use crate::identities::Identity;
-
 pub mod bitwarden;
 pub mod file;
 pub mod prompt;
 
+pub use crate::ports::IdentitySource;
 pub use bitwarden::BitwardenSource;
 pub use file::FileSource;
 pub use prompt::PromptSource;
-
-/// Infra boundary trait: an identity resolver that loads key material from an
-/// external source and returns a concrete Identity.
-pub trait IdentitySource {
-    fn name(&self) -> &str;
-    fn load(&self) -> Result<Box<dyn Identity>, AgeError>;
-}