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.

TODO.md

@@ -19,6 +19,17 @@ or delete the branch.
 
 # Completed Steps
 
+- 2026-08-09: Closed the gap between `make lint` and CI (issue #78).
+  `script/lint` now runs the digest-pinned `golangci-lint` image taken
+  from the `Dockerfile` lint stage, which is the single source of truth
+  for the linter version; the duplicate pin in the `Makefile` `deps`
+  target and the unpinned `golangci-lint` install in `script/bootstrap`
+  are gone. A `golangci-lint` on `PATH` is used only when its version is
+  exactly the pinned one (which is how the lint stage runs it inside the
+  container); anything else goes through Docker, and a missing or
+  unreachable Docker daemon is a hard error rather than a silent
+  fallback. `make check` is therefore now as trustworthy as
+  `script/cibuild`.
 - 2026-08-09: Finished the lint remediation under the canonical
   `.golangci.yml` (issue #61, which also unblocks issue #59). The
   remaining findings were fixed behavior-preservingly: `wsl_v5`