Run golangci-lint only in docker, on every run (closes #55)

script/lint builds the new Dockerfile.lint, where golangci-lint runs as
a build step. The lint stage is rebuilt on every run, so an unchanged
tree is linted too; the module download stays cached. script/bootstrap
no longer installs golangci-lint. The Dockerfile lint stage calls
golangci-lint directly, since make lint now starts a docker build.
golangci-lint config verify is not run: it fetches its schema live over
unpinned HTTPS.

Model: opus-5-5

README.md

@@ -496,15 +496,18 @@ standard: normalized scripts in `script/` are the entrypoints for the
 development workflow, and the Makefile targets are thin shims that call
 them. We provide:
 
-- `script/bootstrap` — install all dependencies (Go, golangci-lint, Go
-  module download), idempotently
+- `script/bootstrap` — install all dependencies (Go, Go module
+  download), idempotently; golangci-lint is not installed, it runs in
+  docker
 - `script/setup` — make a fresh clone ready for development: runs
   `script/bootstrap`, then `script/install-precommit`
 - `script/projectname` — output the project name (`secret`); used by
   other scripts such as `script/docker`
 - `script/test` — run `go vet` and the test suite (verbose rerun on
   failure)
-- `script/lint` — run `golangci-lint`
+- `script/lint` — run `golangci-lint` in docker only: builds
+  `Dockerfile.lint`, where the linter is a build step that runs on every
+  call, also on an unchanged tree
 - `script/fmt` — format all Go code (writes)
 - `script/fmt-check` — check formatting without writing
 - `script/check` — run `script/test`, `script/lint`, and