P2 slice 5: files in and out

Files reach the books through the server: uploads by hash and size,
then filed; exports (the CPA package, the 1099 worksheet, W-9s) handed
over as downloads from the server, never read off disk by the agent.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

crates/ledger-host/Cargo.toml

@@ -15,6 +15,8 @@ serde_json = "1"
 tokio = { version = "1", features = ["rt-multi-thread", "macros", "io-util", "io-std", "sync", "net", "signal", "time", "process"] }
 reqwest = "0.13"
 base64 = "0.22"
+# `wanderbooks download` checks what arrived against its Repr-Digest.
+sha2 = "0.10"
 tokio-util = { version = "0.7", features = ["rt"] }
 # Agents' tokens are kept in the OS keyring (plan 3.3, 3.5): the Secret
 # Service on Linux. macOS and Windows stores come with P11.