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/handbook/cpa-handoff.md

@@ -1,5 +1,5 @@
 # CPA handoff
 
-- books.export_cpa_package carries every member's and 1099 payee's full TIN from the vault (the K-1 worksheet and the 1099 worksheet), so it needs save_to: a new .zip or a folder outside the books and the import folders.
+- books.export_cpa_package carries every member's and 1099 payee's full TIN from the vault (the K-1 worksheet and the 1099 worksheet), so it needs save_to: a new .zip or a folder outside the books.
 - Tell the human where it was saved; never open, read, attach, or copy the file, and never echo its TINs.
 - tin_gaps lists each member or payee whose TIN it lacks and why: collect them (books.set_tin, books.set_w9) and export again.