feat(package): one attention engine over PackageBooks (U0b)

Needs attention was computed by the viewer alone, while books.start
hand-picked a handful of next steps. The human's list and the agent's
could disagree, and a new item had to be written twice (or, as
books.start showed, not at all).

Move the engine to ledger-package over PackageBooks: one function,
find/find_with(books, today, settings, integrity), reads the ledger, the
journal's event types (an entry's origin), the attachments and their
links, the statements, the reconciliations, and the database check. The
viewer's attention module now only maps Target to Route; books.start
turns the same items into its next steps (why = the item's title, top 12),
so the list the human sees and the steps the agent gets cannot drift.

PackageBooks gains settings, journal, statements, reconciliations and
disk_check, implemented for Store and Snapshot, so both views answer
alike. The W-9, reconciliation and 1099 predicates move beside the items
to keep one copy. ledger-package gains ledger-config and serde_json (the
receipt policy and the tool arguments the items carry).

Tests: view_model.rs and spec_kept.rs pass unchanged; the new
attention_parity.rs compares the viewer and books.start item for item on
the demo books and on a book written entry by entry. just check green.

docs/mcp-interface.md

@@ -124,7 +124,12 @@ settings-and-conventions section says how to use them.
   onboarding step (`books.handbook {section: "contract"}`, when the chart
   is empty or `focus` is `onboarding`); `books.open_items` (an item due
   within 30 days or overdue); `books.backup` (none set up, or one stale,
-  failing, paused or failing its drill); the focus's step (`close`:
+  failing, paused or failing its drill); then Needs attention, by
+  severity, at most 12, each one `{tool, args?, why}` whose `why` is the
+  item's title (the same list the viewer draws: `ledger_package::
+  attention`, so the two can't differ; `books.verify` items are the
+  server's own damage check and are not repeated); then the focus's step
+  (`close`:
   `books.conventions {kind: "close_step"}`; `audit`: `books.verify`); then
   always `books.handbook {section: "contract"}` and `books.digest`.
   `focus` is `daily` (the default), `onboarding`, `close` or `audit`;