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-mcp/tests/http_concurrency.rs

@@ -119,7 +119,8 @@ async fn draft(c: &Client, tag: &str, cents: u64, with_receipt: bool) -> u64 {
             c,
             "books.attach_receipt",
             json!({"entry_id": id, "file_name": format!("{tag}.pdf"),
-                   "mime_type": "application/pdf", "content_base64": b64(tag.as_bytes())}),
+                   "mime_type": "application/pdf", "content_base64": b64(tag.as_bytes()),
+                   "sha256": ledger_store::journal::sha256_hex(tag.as_bytes())}),
         )
         .await
         .unwrap();