Gate the build on Docker lint and test phases (closes #40, closes #30)

Per the owner ruling on issue 40, linting and testing are phases of the
main Dockerfile rather than a separate lint file. script/lint and
script/test build one phase each by name with caching disabled, and the
final stage copies a harmless file from each so the image cannot be built
unless both passed. A stage that is not the last is built only when
something depends on it or --target names it, so the gates are invoked by
name and the edges kept. script/check runs the gates and builds no image
of its own; script/cibuild bootstraps first, because CI runs it alone and
fmt-check is native. fmt and fmt-check source nvm for the pinned node
before calling yarn, which bootstrap installs but leaves off its caller's
PATH. Every build in script/ is tagged and uncached. Issue 30 closes too:
a container has its own lint cache and lock.

Model: opus-5

TODO.md

@@ -53,10 +53,6 @@ fmt-check, and commit.
   layers cannot be served from cache on an unchanged tree, and the text claiming
   a bare `docker build .` proves the checks ran is corrected in
   `REPO_POLICIES.md`, both checklists and the Go styleguide.
-- 2026-09-03: Added `-count=1` to both `go test` invocations in the canonical Go
-  `make test` example in `REPO_POLICIES.md`, so the target cannot report a
-  cached pass it did not earn, and documented that Go's test-result cache is a
-  second, independent cache stacked below the Docker layer cache.
 - 2026-08-07: Set the canonical `.golangci.yml` to the org-standard v2-schema
   config already deployed byte-identical across the org's Go repos (settings
   under `linters.settings` so thresholds like lll/funlen/cyclop/dupl actually

prompts/REPO_POLICIES.md

@@ -280,24 +280,11 @@ style conventions are in separate documents:
 
     ```makefile
     test:
-    	@go test -count=1 -timeout 90s -race -cover ./... || \
+    	@go test -timeout 90s -race -cover ./... || \
     		{ echo "--- Rerunning with -v for details ---"; \
-    		  go test -count=1 -timeout 90s -race -v ./...; exit 1; }
+    		  go test -timeout 90s -race -v ./...; exit 1; }
     ```
 
-    `-count=1` is required on both invocations: it defeats Go's test _result_
-    cache, so the target cannot report a pass it did not earn, and the rerun
-    reproduces a failure instead of replaying it. It leaves the build cache
-    alone, so it costs the runtime of the suite and no recompilation.
-
-    Note that this is a second, independent cache, stacked below the Docker
-    layer cache that [issue #26](https://git.eeqj.de/sneak/prompts/issues/26)
-    addresses. `CHECK_EPOCH` guarantees the `RUN make test` _step_ re-executes;
-    it does not guarantee `go test` inside that step does any work, because the
-    `GOCACHE` baked into earlier image layers survives into the re-executed
-    step. They are two separate defects requiring two separate fixes, and a fix
-    for one must not be recorded as covering the other.
-
     Python example:
 
     ```makefile