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(D-46; the inline decisions marked superseded), STATUS.md, testing.md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/spec/rules.md

@@ -1491,7 +1491,9 @@ don't count toward the three: `sources/unicode-tin-text.md`, Unicode
 worksheets carry every member's and payee's TIN by default) are never
 written to a path: no tool takes one (D-29, P2 slice 6). Each is handed
 over as an export the asking token fetches with `wanderbooks download`
-(kept 15 minutes), never inline (D-44). (Until slice 6 they were
+(kept 15 minutes), never inline (D-44; since D-46 no tool takes or
+gives any file's bytes inline: files come in only by upload and go out
+only by download). (Until slice 6 they were
 written only outside the books dir: DECISION, P0 review, 2026-10-16, so
 none could be filed back in as a receipt and read out again.) No file
 holding a full TIN is filed into the books
@@ -1503,7 +1505,8 @@ before the vault keeps it, so a refused call marks no file; each
 a 1099 worksheet with TINs; a CPA package with TINs and each file in it
 that holds one), shared by every session and every later server for the
 books, and every way a file enters the books (`books.attach_receipt`,
-`books.file_document`, an upload or inline) refuses those exact files,
+`books.file_document`, by upload, the only way since D-46) refuses those
+exact files,
 and refuses any text file (by its type or name, or because it decodes as
 UTF-8 or UTF-16, with a byte-order mark or, NUL-interleaved, without one)
 holding (a) a TIN the vault holds for a party of the