feat(mcp): book on custom accounts via account_override; fix kontoplan settings link (#1608)

* feat(mcp): book on custom accounts via account_override; fix kontoplan settings link

gnubok_categorize_transaction only spoke a 19-category enum mapping to 21
hardcoded BAS accounts, so company-custom accounts (e.g. VMB) were
unreachable from the agent surface even when active in the chart.

- add account_override to gnubok_categorize_transaction with v1 REST
  semantics via a shared helper (lib/bookkeeping/account-override.ts):
  business-side replacement, class-2 auto-VAT drop with the 2610-2649
  moms-line exception, plus a same-account degenerate guard; validated at
  staging and re-validated at commit
- align the gnubok_create_voucher staging gate with the engine's seeding
  semantics: BAS 2026 accounts merely absent from the chart pass (the
  engine backfills them at commit) and the preview lists
  will_activate_accounts with BAS-name fallback; non-BAS unknown and
  inactive accounts still rejected
- stop suggest_categories silently dropping mapping rules whose account
  is outside the fixed category maps; they surface with the rule's own
  account and an explanatory match_reason
- correct the create_account next-step hint (categorize could never use
  the new account before; now true via account_override)
- point the settings "Kontoplan (BAS)" link at /chart-of-accounts and
  redirect the orphaned /bookkeeping?tab=accounts URL (tab removed in
  #850; the deep link never worked after the #854 merge collision)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(mcp): address review findings on account_override

- commit executor rejects a present-but-malformed stored account_override
  loudly instead of degrading to the category default (CodeRabbit major;
  the approver approved a preview showing the override account); with
  commitPendingOperation regression tests
- accountToCategory returns null for unknown income accounts so custom
  income accounts get the same diagnostic as expenses (CodeRabbit minor),
  with income + reason-accumulation tests (CodeRabbit nit)
- pin the class-2 VAT-drop balance invariant with a test through
  buildTransactionEntryLines (Swedish compliance review: gross booking,
  never an unbalanced net + missing VAT leg)
- account_override description asks the agent to state the actual
  affärshändelse in notes when overriding (BFL 5 kap description concern)
- eventBus.clear() in the two new test suites (CodeRabbit minor)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(mcp): never guess a moms leg onto an account_override without explicit VAT intent

Round-2 Swedish compliance finding: the class-2 VAT drop did not cover
margin-scheme (VMB) accounts in class 3/4, which are the override's
flagship use case, so a forgotten vat_treatment attached the category
default standard_25 and booked an ingående-moms deduction on a
transaction where input VAT is not deductible (ML 2023:200).

applyAccountOverride now takes explicit VAT intent (vat_treatment or
vat_amount present) and books GROSS with no auto-VAT line without it:
forgetting the flag under-deducts (lawful), never over-deducts. Both
call sites (MCP staging preview, commit core) derive the flag the same
way; the tool description states the enforced behavior. Deliberate
divergence from v1 REST recorded in DECISIONS.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* chore: move stray decision-log entry to the root DECISIONS.md

The round-2 entry was appended from the wrong working directory and
landed as lib/bookkeeping/__tests__/DECISIONS.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/pending-operations/commit.ts

@@ -43,6 +43,7 @@ import {
   resolveUnsettledStatus,
 } from '@/lib/supplier-invoices/lifecycle'
 import { coerceDimensionsBag } from '@/lib/bookkeeping/dimension-resolver'
+import { ACCOUNT_NUMBER_RE } from '@/lib/invariants/account-number'
 import { isSlpPensionAccount } from '@/lib/bookkeeping/slp-lines'
 import { cancelOrphanedPaymentEntry } from '@/lib/bookkeeping/cancel-orphaned-entry'
 import { runWithActor } from '@/lib/bookkeeping/actor-context-node'
@@ -306,6 +307,23 @@ async function commitCategorizeTransaction(
   // propagation all live in the shared core (lib/transactions/categorize-core.ts)
   // so the bulk-book-inbox executor and the Underlag "Bokför valda" route reuse
   // exactly this logic.
+  // Tamper/drift gate for the explicit business-side account: a PRESENT but
+  // malformed account_override must fail loudly, never degrade to the
+  // category default. The approver approved a preview showing the override
+  // account, so posting anything else would diverge from what was approved.
+  const rawAccountOverride = params.account_override
+  if (
+    rawAccountOverride != null &&
+    !(typeof rawAccountOverride === 'string' && ACCOUNT_NUMBER_RE.test(rawAccountOverride))
+  ) {
+    return {
+      error:
+        'Ogiltigt account_override i den stagade operationen (förväntade 4 siffror). ' +
+        'Avvisa operationen och stagea om kategoriseringen.',
+      status: 400,
+    }
+  }
+
   return categorizeMatchedTransaction(supabase, userId, companyId, txId, {
     category,
     vatTreatment,
@@ -314,6 +332,8 @@ async function commitCategorizeTransaction(
     allowDuplicate: params.allow_duplicate === true,
     // Dimensions PR7: resolved at staging; coerce is the drift/tamper gate.
     dimensions: coerceDimensionsBag(params.dimensions),
+    // Validated against the chart both at staging and inside the core at commit.
+    accountOverride: (rawAccountOverride as string | null | undefined) ?? undefined,
   })
 }