feat(mcp): books.write_off_invoice (P4b S7)

One tool for the bad debt (rules.md B-41): it writes off an invoice that
will not be collected, Dr 6990 Bad Debt Expense / Cr 1200 Accounts
Receivable, the receivable line tagged with the invoice so the document
reads written_off and its figures stay derived (B-38).

The evidence is the point, and the tool takes exactly one of
upload_id (an upload, filed with the write-off) and
existing_attachment_id (one already filed, linked to the write-off's
entry and the invoice); neither or both is refused. amount defaults to
what the invoice owes and may be partial.

Without confirmed it is a preview: the engine runs on a clone (every
refusal the post would give) and the answer is the figures the write-off
would leave, entry_id 0, nothing journaled.

The tool lives in its own group file, tools/write_offs.rs (plan U0a), so
S4.4's document tools and this one land apart; it is destructive, needs
books:write to preview and books:post to post (scopes.rs), and is
documented under "Tools — A/R and A/P". The destructive-tool count, the
tool tables and the journey test's tool count each move by one.

Tests over the wire: write_off_then_recovery, write_off_needs_evidence.

crates/ledger-cli/tests/mcp_journeys.rs

@@ -125,7 +125,7 @@ fn mcp_end_to_end() {
     // Tool listing: the narrow tools; chart is read-only.
     let list = mcp.request("tools/list", json!({}));
     let tools = list["result"]["tools"].as_array().unwrap();
-    assert_eq!(tools.len(), 122, "expected 122 tools, got {}", tools.len());
+    assert_eq!(tools.len(), 123, "expected 123 tools, got {}", tools.len());
     // Files come in as tool input: books.begin_upload, never a local path.
     assert!(tools.iter().any(|t| t["name"] == "books.begin_upload"));
     assert!(!tools.iter().any(|t| t["name"] == "books.import_local_file"));