build: run golangci-lint in a pinned container via script/lint (closes #41)

golangci-lint is no longer invoked on the host anywhere in the repo.
Dockerfile.lint pins golangci/golangci-lint:v2.12.2 by digest and runs
the linter as a build step, so a successful build IS a clean lint, and
`make lint` becomes a thin shim over script/lint. This removes the host
linter install that produced a false green here, where a branch that was
genuinely red with a goconst finding reported "0 issues" off the shared
host cache; a container per run has its own cache and lock.

Two deliberate divergences from the sneak/homoicon reference shape:

  - Two stages rather than one. A cached `deps` stage holds
    `go mod download`, then `FROM deps AS lint` carries the source copy
    and the lint run, and script/lint builds with
    `--no-cache-filter=lint`. Caching of the lint result is explicitly
    waived (a cached build lints nothing), and splitting the stages means
    busting the lint layer does not re-fetch the module cache over the
    network on every run.

  - No `golangci-lint config verify` step. It resolves its JSON schema
    over a live, unpinned HTTPS call: an unpinned network input inside
    the one step whose purpose is a pinned, reproducible gate, and a
    schema-host outage would surface as a red build. `golangci-lint run`
    already fails on a malformed config. The reason is recorded in a
    comment in Dockerfile.lint.

.dockerignore excludes .git only; the lint reads the Go sources,
go.mod/go.sum and .golangci.yml, none of which come from there.

The TODO.md scaffold-exemption note is narrowed rather than dropped:
Dockerfile.lint and script/lint are now permitted and required, while CI
config, REPO_POLICIES.md, an application Dockerfile and any other
script/ entrypoint still are not.

Verified, since a green docker build is the classic false green: two
consecutive script/lint runs on an unchanged tree each showed the
`golangci-lint run` layer executing (9.8s and 7.9s, both "0 issues.")
while the deps layers reported CACHED; a deliberate indent-error-flow
violation failed the build naming that finding and the unused one, and a
revert went clean again. `make check` green.

Makefile

@@ -1,8 +1,10 @@
 # Development convenience targets. This repo is exempt from the standard
-# policy scaffold (no Dockerfile, CI, or REPO_POLICIES.md); this Makefile
-# is only a thin wrapper around the Go toolchain, golangci-lint, and
-# prettier so `make fmt` / `make check` behave the same as in sneak's
-# other repos.
+# policy scaffold (no CI config, no REPO_POLICIES.md, no application
+# Dockerfile) except for the lint container: per sneak's 2026-08-09
+# ruling, linting runs in docker only, so Dockerfile.lint and script/lint
+# are part of this repo. This Makefile is otherwise only a thin wrapper
+# around the Go toolchain and prettier so `make fmt` / `make check` behave
+# the same as in sneak's other repos.
 
 GO_PKGS  := ./...
 MD_FILES := $(shell git ls-files '*.md')
@@ -26,9 +28,11 @@ fmt-check:
 	fi
 	$(PRETTIER) --check $(MD_FILES)
 
-# Run the house linter (config in .golangci.yml).
+# Run the house linter. golangci-lint is never installed on the host: the
+# work happens inside the pinned container built by Dockerfile.lint, and
+# this target is a thin shim over the script that builds it.
 lint:
-	golangci-lint run $(GO_PKGS)
+	./script/lint
 
 # Run the test suite. Quiet on success; on failure, rerun verbosely for the
 # full output and still fail the target (the first run already proved the