Drop the lint-guard shell scanner, keep the Dockerfile.lint checks (closes #121)

The guard test in cmd/vaultik/lintdocker_test.go tried to prove that no script runs the linter outside the container by parsing shell scripts with a hand-written scanner. Four reviews each found another spelling it missed; such a parser cannot be complete, and nobody could follow it in one reading.

The scanner, its helpers and their tests are deleted. The plain Dockerfile.lint assertions stay: the linter image is pinned by digest, config verify runs before run, and the per-run value reaches both steps. TODO.md no longer claims a test proves the property; script/lint is the only lint entry point, and keeping it so is a review matter.

Judgement call: this drops a guard two reviewers asked to harden.

model: claude-opus-4-8 (implementation, review); claude-fable-5-1 (decision, merge)

Dockerfile.lint

@@ -72,11 +72,11 @@ RUN [ -n "$CHECK_EPOCH" ] || exit 1
 # running, and exits 0 reporting `0 issues.` on a tree the real config
 # fails. Demonstrated on this repo at this pin, recorded on
 # https://git.eeqj.de/sneak/vaultik/pulls/114: with a planted
-# over-length line, `script/lint` exits 1 naming the `lll` finding with
-# `linters:` and exits 0 with `linterz:`. A set-but-ineffective config
-# quietly falling back to defaults is precisely the false-green class
-# this gate exists to eliminate, so it must not sit in the gate's own
-# configuration.
+# over-length line, `script/lint` exits 1 naming the `revive` finding
+# with `linters:` and exits 0 with `linterz:`. A set-but-ineffective
+# config quietly falling back to defaults is precisely the false-green
+# class this gate exists to eliminate, so it must not sit in the gate's
+# own configuration.
 #
 # `config verify` catches it, and it does so OFFLINE at this pinned
 # version -- verified, not assumed. Under `docker run --network none`