test(agent): scripted evals follow only next_steps; the surface over the wire (P5)

Agent reason: the envelope is only worth having if an agent can do its work
by following it. crates/ledger-cli/tests/agent_evals/ holds six scripted
transcripts, no model: onboarding to a first post, a receipt-backed post, a
draft that waits for its receipt and posts when it arrives, a reversal, a
reconciliation, a month-end close. agent_evals_follow_only_next_steps plays
each against a real books server: after books.start a step may call only a
tool (or fetch only a prompt) the previous result's next_steps named, with
the arguments the step gave and the ones the agent decides; setup calls (the
chart and the books an agent finds) are not the agent's moves.

agent_surface.rs holds the over-the-wire tests of the rest: the journal in
words (order, subject and type filters, paging, a bad subject refused),
source corrections through the three tools, the prompts rendered against
their files and refused when malformed, the resource templates advertised
and read, books.start v2 whole and under 8 KiB with the self-report in the
access log only, a new book told to onboard, and a mutating call answering
with the head the journal has and a refusal naming its remedy.

Docs: testing.md's P5 section (every test, and the For-P12 paragraph naming
what a mutant would flip), parity.md (no feature moved; one row), and
architecture.md (the envelope and the guards); mcp-interface.md's overview
sections now describe the surface as built.

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

crates/ledger-cli/tests/agent_evals/receipt_backed_post.json

@@ -0,0 +1,96 @@
+{
+  "description": "A post the receipt policy covers: the preview asks for the receipt, the receipt is attached, the entry posts.",
+  "settings": "[company]\nname = \"Eval Co\"\nfiscal_year_start = \"01-01\"\n[books]\ncash_accounts = [\"1000\"]\n",
+  "setup": [
+    {
+      "call": "books.add_account",
+      "args": {
+        "code": "1000",
+        "name": "Cash",
+        "account_type": "asset"
+      }
+    },
+    {
+      "call": "books.add_account",
+      "args": {
+        "code": "3000",
+        "name": "Capital",
+        "account_type": "equity"
+      }
+    },
+    {
+      "call": "books.add_account",
+      "args": {
+        "code": "6000",
+        "name": "Rent",
+        "account_type": "expense"
+      }
+    }
+  ],
+  "steps": [
+    {
+      "call": "books.start",
+      "args": {}
+    },
+    {
+      "expect_next_steps_contains": "books.draft_entry"
+    },
+    {
+      "follow": "books.draft_entry",
+      "with": {
+        "date": "2026-01-10",
+        "description": "January rent",
+        "no_party_reason": "test"
+      }
+    },
+    {
+      "follow": "books.add_line",
+      "with": {
+        "account": "6000",
+        "side": "debit",
+        "amount": "40.00"
+      }
+    },
+    {
+      "follow": "books.add_line",
+      "with": {
+        "account": "1000",
+        "side": "credit",
+        "amount": "40.00"
+      }
+    },
+    {
+      "follow": "books.post_entry",
+      "with": {}
+    },
+    {
+      "expect_next_steps_contains": "books.attach_receipt"
+    },
+    {
+      "follow": "books.attach_receipt",
+      "with": {
+        "upload_id": {
+          "$upload": {
+            "name": "receipt.pdf",
+            "mime": "application/pdf",
+            "text": "%PDF-1.4 a receipt for rent",
+            "purpose": "receipt"
+          }
+        }
+      }
+    },
+    {
+      "follow": "books.post_entry",
+      "with": {}
+    },
+    {
+      "follow": "books.post_entry",
+      "with": {}
+    },
+    {
+      "expect": {
+        "/status": "posted"
+      }
+    }
+  ]
+}