feat(core,mcp,gui): never apply a legal rule past what wanderbooks knows

Stephen (2026-10-13): when what wanderbooks knows is stale, check the
date, research it or ask the owner or CPA, and update the rule for
everyone with a pull request (best) or an issue (worst).

- ledger_core::rules: a registry of rules set by law that change over
  time. Each says the years it's known for, its rules.md entry, its
  vendored official source, and a review date. stale_rules(today) is
  pure; the engine has no clock.
- The 1099 threshold is the first: $600 through 2025, $2,000 in 2026,
  then indexed for inflation (26 U.S.C. §6041(h)). default_threshold
  and vendor_1099_report now return Result; for 2027+ they refuse with
  LedgerError::RuleNotKnown, whose message says what to do: get the
  figure from the official source or the owner/CPA, pass it explicitly,
  then open a PR (or issue) at github.com/swaits/wanderbooks. Before,
  2027+ silently used $2,000.
- MCP: books.vendor_1099 relays that refusal; books.digest gains
  stale_rules; the server instructions tell the agent the procedure (and
  how to handle control-account conflicts: explain, never renumber).
  The CPA package writes a NOT-INCLUDED note instead of a guessed 1099
  worksheet.
- Viewer: "Needs attention" shows each stale rule with a ready prompt
  for the agent; the 1099 page explains instead of guessing.

Review date for the 1099 rule: 2026-11-01, when the IRS usually
publishes next year's inflation adjustments (CONVENTION, marked as such
in the registry).

Tests: an_unknown_year_threshold_is_never_guessed,
dated_rules_go_stale_on_their_review_date (dates passed in, so no clock
in the test), MCP stdio: 2027 refused with guidance, explicit threshold
accepted, digest carries stale_rules (asserted date-independently).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

docs/mcp-interface.md

@@ -107,9 +107,13 @@ below.
   accounts (required); the three sections always sum to their net change.
   Disposal gains/losses move from operating to investing automatically.
 - `books.digest { since: "2026-10-01" }`
-  → `{ entries_posted: [...], totals_by_account: [...] }`. The human review
-  surface: "what did the AI do with my books lately." The digest is the
-  answer to "trust, but verify."
+  → `{ entries_posted: [...], totals_by_account: [...], stale_rules: [...] }`.
+  The human review surface: "what did the AI do with my books lately." The
+  digest is the answer to "trust, but verify." `stale_rules` lists legal
+  amounts wanderbooks may no longer know as of today (`{ name,
+  known_through, catalog, source, why }`): research the current value or
+  ask the human, then update it upstream (pull request or issue at
+  github.com/swaits/wanderbooks).
 
 ## Tools — fixed assets
 
@@ -220,7 +224,9 @@ whether a W-9 is on file.
   takes `vendor` too; bills and their payments are tagged automatically.
 - `books.vendor_1099 { year, threshold? }` → `{ year, threshold, vendors:
   [{ vendor_id, name, form_1099?, tin_on_file, paid, reportable }],
-  missing_w9, notes }`.
+  missing_w9, notes }`. For a year whose threshold wanderbooks doesn't know
+  (after 2026 until the indexed amount is added) it refuses with what to
+  do; pass `threshold` explicitly once the figure is confirmed.
 
 ## Tools — receipts & documents