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/src/bridge.rs

@@ -59,8 +59,8 @@ use tokio::io::{AsyncBufReadExt, AsyncReadExt, AsyncWriteExt};
 use tokio::sync::{Notify, mpsc, watch};
 use tokio::time::Instant;
 
-/// The longest message the agent may send (receipts ride in calls until
-/// uploads have their own route).
+/// The longest message the agent may send (a file bigger than a call
+/// takes inline goes by an upload).
 const MAX_MESSAGE: usize = ledger_mcp::local::MAX_BODY;
 /// The bridge's own request ids upstream start so.
 const OWN_ID: &str = "wanderbooks-bridge-";