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-cli/tests/spec_mcp.rs

@@ -1294,6 +1294,8 @@ fn happy_path(name: &str) -> Mcp {
     // The access log, the backups' status, and a backup now (none is set
     // up: it says so).
     m.ok("books.access_log", json!({}));
+    // P5: the journal in words.
+    m.ok("books.journal_history", json!({"limit": 5}));
     m.ok("books.backup", json!({}));
     m.ok("books.back_up_now", json!({}));