test(good-fences): lend-the-mower journey (red)

The spec is a sequence diagram of the a2030.md story: I lend a governed
mower; the neighbor's harness shows the model only `about`, runs its
code as a tenant, and the governor stops the tipsy tenant at a 2-cell
fence. The test runs the actors through a causal tracer and compares
the trace with the diagram, then checks the yard and what the model saw.

The tracer comes from agoric-sdk PR #12895, extended to name
participants made mid-sequence (governor, tenant) and to draw thrown
errors as replies.

Red: src/ doesn't exist yet (ERR_MODULE_NOT_FOUND for src/fence.ts).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

good-fences/docs/lend-the-mower.md

@@ -0,0 +1,63 @@
+# Lend the mower
+
+The journey from [Good Fences Make Good Agents](../../../a2030.md): I lend
+my neighbor a governed mower; the neighbor's tenant, a little tipsy,
+pushes it until the fence stops it. The design decisions behind each actor are in that
+doc's Design IBIS (numbers in parentheses below).
+
+Actors, by authority:
+
+- **fence page**, **js playground**, **governed mower**, and **yard** run
+  in my browser (3a, 8b, 6b).
+- **Mower factory** trades a webkey for a mower (3b-ii); each **mower DO**
+  is given the yard and cuts the cells it enters (6a, 6d).
+- The **neighbor's harness** holds the things I send and shows the
+  **model** only their `about` (1b, 2a); it runs the model's code as a
+  **tenant**, endowed with the things (2c, 5a).
+
+The yard is 8×8 here and the fence is 2 cells, to keep the diagram short
+(the story's 10 cells is 6c). The mower starts at (4, 4), facing north.
+
+## Lend a governed mower to a tipsy tenant
+
+```mermaid
+sequenceDiagram
+  %% ->> is my action at the fence page; -->> is a call or reply it causes.
+  participant F as fence page
+  participant MF as Mower factory
+  participant P as js playground
+  participant G as governed mower
+  participant M as mower DO
+  participant Y as yard
+  participant N as neighbor's harness
+  participant L as model
+  participant TL as tenant loader
+  participant T as tenant
+  F->>F: lend({ webkey: 'mower-key', limit: 2, say })
+  F-->>MF: mower('mower-key')
+  MF-->>F: mower
+  F-->>P: govern(mower, 2)
+  P-->>F: governed
+  F-->>N: chat({ role, content, things: { mower }, about: { mower } })
+  N-->>L: complete([{ role, content, about: { mower } }])
+  L-->>N: code
+  N-->>TL: load(code, { mower })
+  TL-->>N: tenant
+  N-->>T: run()
+  T-->>G: start()
+  G-->>M: start()
+  M-->>Y: cut(4, 4)
+  T-->>G: push()
+  G-->>M: push()
+  M-->>Y: cut(4, 3)
+  T-->>G: push()
+  G-->>M: push()
+  M-->>Y: cut(4, 2)
+  T-->>G: push()
+  G-->>T: throws 'fence: 2 cells from start'
+  T-->>N: 'fence: 2 cells from start'
+  N-->>F: 'fence: 2 cells from start'
+```
+
+The model never sees the mower itself, only `about.mower`. The third
+`push()` never reaches the mower DO: the governor refuses it (7b).