fix(mcp): a read says only what follows; a success adds no empty warnings (P5)

Two existing contracts the envelope broke when every result got every field:
a resource and its tool say the same (books://settings is books.settings,
checked by the_kept_resources_read), and a tool with no warnings has no
warnings key (books.add_account, defect 8). So head and next_steps are on
every tool that can change the books, next_steps is on a read only when
something follows it, guards is on a refusal, and the envelope adds no
warnings of its own (a tool's stay as they are).

The happy-path spec test now exercises books.journal_history (it fails on
a tool it never called), the demo company gets its books.start size check
(Done when 3), and the As-built block of P5 is written.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

crates/ledger-mcp/src/next_steps.rs

@@ -12,7 +12,10 @@
 //!   1. The envelope is applied in one place (`apply`, called by
 //!      `call_tool`), so no tool can forget it.
 //!   2. Every result of a tool that is not read-only has a `head` (the
-//!      journal after the call's last write) and at least one next step.
+//!      journal after the call's last write) and at least one next step; a
+//!      read has `next_steps` when something follows it and nothing else (a
+//!      resource and its tool agree). `guards` is on a refusal, and a
+//!      success without warnings has no `warnings` key.
 //!   3. A tool's own fields are never overwritten: a payload that already
 //!      has `next_steps`, `guards`, `warnings` or `head` keeps it.
 //!   4. A refused call has no head and no steps; a refusal that is a
@@ -451,14 +454,18 @@ pub(crate) fn apply(
     if mutating && steps.is_empty() {
         steps.push(digest_step());
     }
-    if !body.contains_key("next_steps") {
+    // A tool that can change the books always says what follows; a read
+    // says it only when something does, so what it says is what a resource
+    // of the same name says (`books://settings` is `books.settings`).
+    // `guards` is on a refusal (above) and `warnings` is the tool's own:
+    // a success that has none says none, as the tools always did.
+    if !body.contains_key("next_steps") && (mutating || !steps.is_empty()) {
         body.insert(
             "next_steps".into(),
             Value::Array(steps.iter().map(Step::to_json).collect()),
         );
     }
-    body.entry("guards").or_insert_with(|| json!([]));
-    body.entry("warnings").or_insert_with(|| json!([]));
+
     if let Some(head) = head {
         body.entry("head").or_insert(head);
     }

crates/ledger-mcp/src/server_tests.rs

@@ -1564,8 +1564,8 @@ fn every_mutating_result_has_head_and_next_steps() {
         };
         let body = r.structured_content.expect("structured");
         assert!(
-            body["guards"].is_array() && body["warnings"].is_array(),
-            "{name}"
+            body.get("guards").is_none() && body.get("warnings").is_none(),
+            "{name}: a success without them says none"
         );
         let ContentBlock::Text(t) = &r.content[0] else {
             panic!("{name}: no text block");