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

`script/lint` ran whatever `golangci-lint` was on `PATH` while CI ran
the digest-pinned image from the `Dockerfile` lint stage. The two
versions disagree about real findings, so `make check` could be green
on a tree CI fails - and, on this host's 2.10.1, red on a tree CI
passes. A gate that can differ from CI is not a gate.

`script/lint` now runs the pinned image itself. The single source of
truth for the linter version is the `Dockerfile` lint stage `FROM`
line: `script/lint` parses the image reference (tag AND digest) out of
it with awk and runs exactly that image, so bumping the linter is a
one-line edit there and nowhere else. The duplicate pin in the
`Makefile` `deps` target (`go install ...@v2.12.2`) and the unpinned
`golangci-lint` install in `script/bootstrap` are removed rather than
kept in sync: with linting containerized, a second copy on `PATH` is
only a way to drift.

A `golangci-lint` on `PATH` is used only when its version is exactly
equal to the pin - the same binary by definition, and the case that
matters is the lint stage itself, which runs `make lint` inside the
pinned container where no Docker daemon exists. Every other version
goes through Docker, and a missing or unreachable daemon is a hard
error naming the required image, never a silent fallback.

The container run mounts persistent `GOCACHE`, `GOMODCACHE` and
`GOLANGCI_LINT_CACHE` directories under `${XDG_CACHE_HOME:-~/.cache}`
and runs as the invoking uid/gid, so repeat runs stay fast (2.7s warm
vs 1.8s for the ambient binary) and nothing lands root-owned.
`script/lint-fix` delegates to `script/lint --fix` so autofixes come
from the same pinned linter.

README documents that `make check` is authoritative because of this,
and points at `script/cibuild` as the full CI-equivalent gate.

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