Run all linting in Docker via Dockerfile.lint (closes #109)

golangci-lint no longer runs on the host. script/lint builds
Dockerfile.lint, which copies the repo into the digest-pinned
golangci-lint image and lints as a build step, so a successful build is
a clean lint. The host binary shared one cache and one lock with every
other checkout on the machine, which produced findings attributed to
unrelated worktrees as well as unearned passes.

Three properties the wrapper has to get right:

- --no-cache-filter=lint forces the lint stage to re-execute. Without
  it an unchanged tree replays the layer and the build exits 0 in under
  a second having linted nothing. The deps stage stays cacheable.
- docker silently ignores --no-cache-filter when the stage name does
  not match, so the flag alone is a convention, not a guarantee: a
  rename or a typo restores the cached false green with no warning.
  script/lint therefore tees the build output and fails unless
  golangci-lint's own summary line ("N issues." / "N issues:") appears
  in it. No summary, no lint, whatever the exit code says.
- Both lint steps use RUN --network=none. golangci-lint config verify
  is documented as fetching its JSON schema over HTTPS; the pinned
  image resolves it with no network, and --network=none enforces that
  rather than trusting it. Verify is kept because golangci-lint run
  silently ignores config keys it does not recognize.

The main Dockerfile's lint stage now invokes golangci-lint directly
instead of `make lint`, which would otherwise need a docker daemon
inside the build.

golangci-lint installation is removed from script/bootstrap. Its curl
guard and its script/fetch-assets call are untouched.

script/bootstrap

@@ -3,20 +3,14 @@
 # this repo. Idempotent: every install is guarded by a check so already
 # installed tools are skipped. Base tooling comes from nix, apt, brew,
 # or apk (detected in that order); assumes NOTHING is present (not git,
-# make, or go). golangci-lint is packaged in nix, brew, and apk; on apt
-# it is installed from a hash-verified GitHub release archive (never
-# curl | sh). Finishes by running script/fetch-assets, which installs the
-# hash-pinned third-party browser assets the repo does not commit.
+# make, or go). golangci-lint is deliberately not installed: linting runs
+# only in docker, via script/lint and Dockerfile.lint. Finishes by running
+# script/fetch-assets, which installs the hash-pinned third-party browser
+# assets the repo does not commit.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# Pinned versions, 2026-08-07. Never "latest"; exact versions only.
-GOLANGCI_LINT_VERSION="2.12.2"
-# sha256 of golangci-lint-2.12.2-linux-<arch>.tar.gz release archives
-GOLANGCI_LINT_SHA256_AMD64="8df580d2670fed8fa984aac0507099af8df275e665215f5c7a2ae3943893a553"
-GOLANGCI_LINT_SHA256_ARM64="44cd40a8c76c86755375adfeea52cfd3533cb43d7bd647771e0ae065e166df3a"
-
 PKGMGR=""
 SUDO=""
 
@@ -57,52 +51,6 @@ missing() {
     ! command -v "$1" >/dev/null 2>&1
 }
 
-# verify_sha256 <file> <expected-hash>
-verify_sha256() {
-    if command -v sha256sum >/dev/null 2>&1; then
-        actual="$(sha256sum "$1" | cut -d' ' -f1)"
-    else
-        actual="$(shasum -a 256 "$1" | cut -d' ' -f1)"
-    fi
-    if [ "$actual" != "$2" ]; then
-        echo "bootstrap: sha256 mismatch for $1" >&2
-        echo "  expected: $2" >&2
-        echo "  actual:   $actual" >&2
-        exit 1
-    fi
-}
-
-# apt has no golangci-lint package: install a pinned release archive
-# from GitHub, verified by hardcoded sha256 (never curl | sh).
-install_golangci_lint_release() {
-    case "$(uname -m)" in
-        x86_64) goarch="amd64"; sha="$GOLANGCI_LINT_SHA256_AMD64" ;;
-        aarch64|arm64) goarch="arm64"; sha="$GOLANGCI_LINT_SHA256_ARM64" ;;
-        *)
-            echo "bootstrap: unsupported architecture $(uname -m)" >&2
-            exit 1
-            ;;
-    esac
-    if missing curl; then pkg_install curl curl curl curl; fi
-    name="golangci-lint-${GOLANGCI_LINT_VERSION}-linux-${goarch}"
-    tmp="$(mktemp -d)"
-    curl -fsSL -o "$tmp/$name.tar.gz" \
-        "https://github.com/golangci/golangci-lint/releases/download/v${GOLANGCI_LINT_VERSION}/${name}.tar.gz"
-    verify_sha256 "$tmp/$name.tar.gz" "$sha"
-    tar -xzf "$tmp/$name.tar.gz" -C "$tmp"
-    $SUDO install -m 0755 "$tmp/$name/golangci-lint" /usr/local/bin/golangci-lint
-    rm -rf "$tmp"
-}
-
-ensure_golangci_lint() {
-    if ! missing golangci-lint; then return 0; fi
-    detect_pkgmgr
-    case "$PKGMGR" in
-        apt) install_golangci_lint_release ;;
-        *) pkg_install golangci-lint golangci-lint golangci-lint golangci-lint ;;
-    esac
-}
-
 main() {
     cd "$ROOT"
 
@@ -110,9 +58,14 @@ main() {
     if missing git; then pkg_install git git git git; fi
     if missing make; then pkg_install gnumake make make make; fi
 
-    # Go toolchain and linter
+    # Go toolchain
     if missing go; then pkg_install go golang go go; fi
-    ensure_golangci_lint
+
+    # Not installed here: docker is platform-specific and out of scope for a
+    # package-manager bootstrap, but script/lint needs it.
+    if missing docker; then
+        echo "bootstrap: docker not found; script/lint requires it" >&2
+    fi
 
     go mod download