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/spec_assets.rs

@@ -360,7 +360,9 @@ impl Mcp {
         self.ok(
             "books.file_document",
             json!({"category": "legal", "file_name": "bill-of-sale.txt", "mime_type": "text/plain",
-                   "content_base64": b64(b"bill of sale"), "description": "Bill of sale"}),
+                   "content_base64": b64(b"bill of sale"),
+                   "sha256": ledger_store::journal::sha256_hex(b"bill of sale"),
+                   "description": "Bill of sale"}),
         )["attachment_id"]
             .as_u64()
             .unwrap()