Stop the vault tools promising verification they don't perform

Three findings from the automated review passes, all confirmed against the
code before changing anything.

The download URL was dead on arrival for the caller it was built for. Sure
serves stored files through Active Storage controllers that
config/initializers/active_storage_authorization.rb gates on
`viewable_by?(Current.user)` — a signed-in browser session. An MCP client has
a bearer token and no session, so following the URL would have redirected to
sign-in. Removed it rather than leaving a link that cannot work, and the
description now points at search_family_files or the vault UI.

Coverage called a month `covered` when a document merely existed. An
unreconciled statement is not mismatched, so it took the `covered` branch, and
the payload carried nothing to correct the reading — the same "advertised
verification that never happened" bug fixed last round in
get_account_statement, in a second place. Months now carry their own
reconciliation_status, and the description says covered means presence, not
agreement.

Listing filtered visibility after limiting. Beyond underfilling a page, with
no cursor and a 100-row cap an accessible statement behind enough newer
invisible ones was unreachable. Visibility now lives in the query, mirroring
viewable_by? for a statement manager.

Also: rescue unexpected upload failures into a tool error instead of a raw
exception string, derive the documented size limit from MAX_FILE_SIZE, list
every coverage status in mcp.md, and cover the failed-reconciliation and
base64-normalisation branches.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JFDp9HhXDeswadu4cxFojn

test/models/assistant/function/get_statement_coverage_test.rb

@@ -32,6 +32,25 @@ class Assistant::Function::GetStatementCoverageTest < ActiveSupport::TestCase
     assert_equal "missing", february[:status]
   end
 
+  # "covered" means a document exists, not that it agrees with the ledger — an
+  # unreconciled statement still counts as covered, so the month has to carry its
+  # own reconciliation status or the agent cannot tell the two apart.
+  test "a covered month reports unavailable reconciliation when no balances are entered" do
+    january = Date.current.prev_year.beginning_of_year
+    statement = AccountStatement.create_from_upload!(
+      family: @user.family,
+      account: @account,
+      file: uploaded_file(filename: "unreconciled.csv", content_type: "text/csv", content: "date,amount\n2024-01-01,1\n")
+    )
+    statement.update!(period_start_on: january, period_end_on: january.end_of_month)
+
+    result = @function.call("account_id" => @account.id, "year" => january.year)
+    covered = result[:months].find { |m| m[:month] == january.strftime("%Y-%m") }
+
+    assert_equal "covered", covered[:status]
+    assert_equal "unavailable", covered[:reconciliation_status]
+  end
+
   test "rejects a non-uuid account id" do
     result = @function.call("account_id" => "nope")