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.

Dockerfile

@@ -1,4 +1,10 @@
 # Lint stage
+#
+# This FROM line is the single source of truth for the linter version:
+# script/lint parses the image reference out of it and runs that exact
+# image, so a local `make lint` and CI use the same linter. Bump the
+# linter here (tag AND digest) and nowhere else.
+#
 # golangci/golangci-lint:v2.12.2-alpine, 2026-08-07
 FROM golangci/golangci-lint:v2.12.2-alpine@sha256:91b27804074a0bacea298707f016911e60cf0cdbc6c7bf5ccacb5f0606d18d60 AS lint