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/prompt.rs

@@ -1,7 +1,7 @@
 use crate::error::AgeError;
 use crate::identities::Identity;
 use crate::identities::scrypt::ScryptIdentity;
-use crate::sources::IdentitySource;
+use crate::ports::IdentitySource;
 
 pub struct PromptSource;