refactor!: files only by handle: uploads in, downloads out (D-46)

The owner (2026-10-23, via the question tool): "Uploads only"; and
before it: "What we would DEFINITELY want to avoid is passing a bunch
of BLOBs through AI context windows. That's just a pure waste." No file
passes through an agent's context, in either direction.

In: the inline arguments are gone with the 256 KiB limit (INLINE_MAX,
take_file's inline branch): content_base64 and its sha256 on
books.attach_receipt and books.file_document, w9_content_base64 and
w9_sha256 on books.set_w9, csv_text on the two CSV imports. A file comes
only by books.begin_upload and `wanderbooks upload`, named by its
upload_id; existing_attachment_id still names bytes stored already. The
five filing tools deny unknown fields, and any inline argument to any
tool is refused before it runs as an unknown argument pointing to
books.begin_upload (access log: refused inline_file).

Out: books.export (every format), books.asset_labels and
books.get_receipt view bytes hand the file over (HandedFile: download
command and expiry, D-29), as the CPA package and the W-9 do. Resource
reads stay text. A token keeps up to 64 handed files (was 16). The
access log's digest no longer exempts file bytes.

Docs: handbook files-in-out and parties-and-tins, mcp-interface.md,
rules.md, receipt-store.md, parties-and-attachments.md, plan/served.md
(D-46; the inline decisions marked superseded), STATUS.md, testing.md
(D-46: tests migrated to uploads and downloads, rewritten, removed,
added).

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

docs/receipt-store.md

@@ -93,8 +93,8 @@ a backup is then proven where it lands, by a drill (docs/mcp-interface.md,
 - **Deduplicated.** Attaching the same receipt twice stores one blob and two
   attachment rows. `attach_receipt` / `file_document` return
   `deduplicated: true` when the blob already existed.
-- **Limits.** 25 MiB per file as an upload (`books.begin_upload`), 256
-  KiB given inline. The name and MIME type are the client's, checked
+- **Limits.** 25 MiB per file, as an upload (`books.begin_upload`): the
+  only way a file comes in (D-46: nothing is given inline). The name and MIME type are the client's, checked
   when the file is filed: a name of 1 to 255 bytes, no `/` or `\`, not
   `.` or `..`, no control or invisible characters; a `type/subtype` of
   visible ASCII, at most 127 bytes (`docs/mcp-interface.md`, "Tools —