books.renumber_account: give an account a new code, history kept

Ledger::renumber_account changes only the code (entries name accounts by
id): refuses an unknown account, a blank or used code, and a control code
for an account of the wrong type or a placeholder. Journaled as
account.renumbered (with the previous code), replayed, projected, shown
in the viewer's journal. The MCP tool lists wanderbooks.toml settings
that still name the old code. The server instructions now warn that the
engine can't tell a same-type account with another purpose at a control
code (a hosting expense at payroll's 6100).

Tests: core property (only the code changes; trial balance equal), core
refusals, the store's command property, MCP end to end with restart.
Mutation on the new code: 8 of 8 viable caught.

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

crates/ledger-mcp/tests/mcp_stdio.rs

@@ -147,7 +147,7 @@ fn mcp_end_to_end_over_stdio() {
     // Tool listing: 23 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(), 66, "expected 66 tools, got {}", tools.len());
+    assert_eq!(tools.len(), 67, "expected 67 tools, got {}", tools.len());
     let chart = tools.iter().find(|t| t["name"] == "books.chart").unwrap();
     assert_eq!(chart["annotations"]["readOnlyHint"], true);
     assert!(
@@ -1900,3 +1900,126 @@ fn bank_import_reads_dates_rules_and_files_whole() {
         "the first row was journaled"
     );
 }
+
+/// books.renumber_account moves an account off a control code: payroll then
+/// gets its own wages account, the renumbered account keeps its entries,
+/// and settings naming the old code are listed.
+#[test]
+fn renumbering_an_account_keeps_its_history() {
+    let dir = test_dir("renumber");
+    let toml = dir.join("wanderbooks.toml");
+    std::fs::write(
+        &toml,
+        "[books]\nreceipt_threshold = \"1000000.00\"\ncash_accounts = [\"1000\"]\n",
+    )
+    .unwrap();
+    let mut mcp = Mcp::spawn_with(
+        &dir.join("books-data"),
+        &[std::ffi::OsStr::new("--books-toml"), toml.as_os_str()],
+    );
+    mcp.request(
+        "initialize",
+        json!({
+            "protocolVersion": "2025-06-18",
+            "capabilities": {},
+            "clientInfo": {"name": "smoke-test", "version": "0.1.0"},
+        }),
+    );
+    mcp.notify("notifications/initialized");
+    for (code, name, typ) in [
+        ("1000", "Bank", "asset"),
+        ("2100", "Due to owners", "liability"),
+        ("6100", "Domain & Hosting", "expense"),
+    ] {
+        mcp.call(
+            "books.add_account",
+            json!({"code": code, "name": name, "account_type": typ}),
+        );
+    }
+    let e = mcp.call(
+        "books.draft_entry",
+        json!({"date": "2026-03-05", "description": "domain"}),
+    );
+    let id = e["entry_id"].as_u64().unwrap();
+    for (account, side) in [("6100", "debit"), ("2100", "credit")] {
+        mcp.call(
+            "books.add_line",
+            json!({"entry_id": id, "account": account, "side": side, "amount": "12.34"}),
+        );
+    }
+    mcp.call(
+        "books.post_entry",
+        json!({"entry_id": id, "confirmed": true}),
+    );
+
+    // Refusals: a code in use; a control code of another type (2110 is
+    // payroll's federal withholding, a liability).
+    let err = mcp.call_err(
+        "books.renumber_account",
+        json!({"account": "6100", "code": "1000"}),
+    );
+    assert!(err.contains("already in use"), "{err}");
+    let err = mcp.call_err(
+        "books.renumber_account",
+        json!({"account": "6100", "code": "2110"}),
+    );
+    assert!(err.contains("2110"), "{err}");
+
+    let out = mcp.call(
+        "books.renumber_account",
+        json!({"account": "6100", "code": "6050"}),
+    );
+    assert_eq!(out["previous_code"], "6100");
+    assert_eq!(out["code"], "6050");
+    assert_eq!(out["config_mentions"], json!([]));
+    let chart = mcp.call("books.chart", json!({}));
+    let acct = chart["accounts"]
+        .as_array()
+        .unwrap()
+        .iter()
+        .find(|a| a["code"] == "6050")
+        .expect("renumbered");
+    assert_eq!(acct["name"], "Domain & Hosting");
+    assert_eq!(acct["balance"], "12.34");
+    assert!(
+        !chart["accounts"]
+            .as_array()
+            .unwrap()
+            .iter()
+            .any(|a| a["code"] == "6100")
+    );
+    // The cash account is named in wanderbooks.toml: listed.
+    let out = mcp.call(
+        "books.renumber_account",
+        json!({"account": "1000", "code": "1010"}),
+    );
+    assert_eq!(out["config_mentions"], json!(["[books] cash_accounts"]));
+    drop(mcp);
+
+    // A restart replays the journal to the same chart.
+    let mut mcp = Mcp::spawn(&dir.join("books-data"));
+    mcp.request(
+        "initialize",
+        json!({
+            "protocolVersion": "2025-06-18",
+            "capabilities": {},
+            "clientInfo": {"name": "smoke-test", "version": "0.1.0"},
+        }),
+    );
+    mcp.notify("notifications/initialized");
+    let chart = mcp.call("books.chart", json!({}));
+    let codes: Vec<&str> = chart["accounts"]
+        .as_array()
+        .unwrap()
+        .iter()
+        .map(|a| a["code"].as_str().unwrap())
+        .collect();
+    assert!(
+        codes.contains(&"6050") && codes.contains(&"1010"),
+        "{codes:?}"
+    );
+    assert!(
+        !codes.contains(&"6100") && !codes.contains(&"1000"),
+        "{codes:?}"
+    );
+}