Record the two conventions the live v2 bugs produced

The history write's context.WithoutCancel reads like a mistake if you don't know
why it's there, and "tidying" it is exactly how the orphan-history bug came back
a second time (#73). Written down so the next person — or the next session —
doesn't remove it on the way past.

Also a short testing section. Two of the three bugs only real use found were
invisible to the tests I had: a route that was never registered (service tests
can't see that) and a fixture that populated a field the real response leaves
empty (a test asserting my mental model rather than the API).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

CLAUDE.md

@@ -92,6 +92,29 @@ Frontend: React 19 + Vite + Tailwind 4 + TanStack Router/Query, built into
 - **Every service mutation lands in history** (#48). If you add one, record it —
   see `internal/service/revisions.go`. Multi-row operations pass all their
   changes to a single `record` call so they undo as one unit.
+- **The history write is detached from cancellation on purpose.** `commitScope`
+  calls `context.WithoutCancel` — that is not a mistake to tidy up. By the time
+  a commit runs, the rows it describes are already written, so cancelling it
+  cannot undo anything; it can only leave real changes with no way to undo them.
+  This was a live bug twice (#73): a client disconnect mid-request orphaned 18
+  plantings. Fixing it per-call-site is how it came back, which is why the rule
+  lives in `commitScope` where no caller can forget it.
+
+## Testing
+
+Match the test to the failure it would catch:
+
+- **Anything addressed by its own id needs an API-level test through the router.**
+  Service tests can't see a route that was never registered — PATCH/DELETE
+  `/journal/:id` once shipped fully implemented, fully unit-tested, and
+  completely unreachable.
+- **Watch for fixtures that assert your assumptions instead of the API.** A test
+  for the undo message passed because the fixture I wrote populated a field the
+  real response leaves empty. If a test builds the thing it's testing against,
+  it is checking your mental model, not the system.
+- Some things only real use finds. The agent's whole loop is covered by
+  majordomo's scriptable fake provider (`provider/fake`), which is worth using —
+  but the three worst v2 bugs all turned up in one live session afterwards.
 
 ## Workflow