Run the linter at the pinned version locally too (closes #78)

script/lint ran bare golangci-lint from PATH while CI and the Dockerfile
pinned v2.12.2 by digest, so make lint and CI could disagree about
findings. That drift ran both directions: it produced two false green
claims during the lint remediation, and on an ambient 2.10.1 it also
reported four gosec findings on a tree CI linted clean.

script/lint now extracts the image reference - tag and digest - from the
Dockerfile lint stage FROM line and runs that exact image under docker.
The Dockerfile FROM line is the single source of truth for the linter
version; the duplicate pins in the Makefile deps target and in
script/bootstrap are removed rather than kept in sync.

A golangci-lint on PATH is used only when its version exactly equals the
pin, which is what makes the in-container lint stage work (the Dockerfile
runs make lint inside the pinned image, where there is no docker daemon).
Any other version, or none, goes through docker. When docker is
unavailable the script fails with an actionable message and never falls
back to a different linter version.

script/lint-fix delegates to script/lint --fix so autofixes come from the
pinned linter too. The container mounts persistent build and module
caches and runs as the invoking uid/gid.

Verified by reinstating the four historical nolint directives that 2.10.1
requires and 2.12.2 reports as unused: the old script passed on that tree
and the new one fails with four nolintlint findings.

script/bootstrap

@@ -58,11 +58,13 @@ main() {
     # Go toolchain
     if missing go; then pkg_install go golang go go; fi
 
-    # golangci-lint: packaged in nix, brew, and apk. There is no apt
-    # package; on apt systems install it manually from a hash-verified
-    # GitHub release archive (never curl | sh).
-    if missing golangci-lint; then
-        pkg_install golangci-lint golangci-lint golangci-lint golangci-lint
+    # golangci-lint is deliberately NOT installed: script/lint runs the
+    # digest-pinned golangci-lint image from the Dockerfile's lint stage,
+    # so whatever a package manager happens to ship would only be a
+    # shadow of the pinned version that could drift from CI.
+    if missing docker; then
+        echo "bootstrap: docker not found; script/lint needs it to run" >&2
+        echo "bootstrap: the pinned linter (see the Dockerfile lint stage)" >&2
     fi
 
     # sqlite3 CLI: the test suite shells out to it (VACUUM).