build: always install pinned lint tools in script/bootstrap (closes #117)

`script/bootstrap` guarded its pinned `go install` calls with
`missing()`, which only tests whether a binary is on `PATH`. On any
machine that already had some `golangci-lint`, the install was skipped
and the commit pin had no effect: a v1.x binary cannot parse this
repo's v2-schema `.golangci.yml`, and a different v2.x can silently
disagree with CI. The same reasoning made the v2.12.2 pin bump inert
on every already-provisioned machine.

Install both pinned tools unconditionally. `go install` at a fixed
commit ref is idempotent and cheap with a warm module cache, so
skipping it saved nothing. The `missing()` presence check is kept for
`git`, `make`, and `go`, which really are system-package presence
checks.

Also warn when `PATH` resolves either tool somewhere other than the
directory `go install` writes to, since a shadowing copy earlier on
`PATH` is what `make lint` and `make fmt` would actually run. This is
a warning, not a failure: the remedy is the user's `PATH`.

The pins themselves are unchanged and still match the Dockerfile.

TODO.md

@@ -25,6 +25,13 @@ confirm make check still passes.
 
 # Completed Steps
 
+- 2026-08-09: `script/bootstrap` now installs the pinned `golangci-lint`
+  and `goimports` unconditionally instead of only when the binary is
+  absent from `PATH`, so the commit pins actually take effect on
+  already-provisioned machines; it also warns when `PATH` resolves
+  either tool to a copy outside the directory `go install` writes to.
+  The `missing` presence check is retained for `git`, `make`, and `go`
+  (#117)
 - 2026-08-07: golangci-lint bumped to v2.12.2 (commit-pinned installs
   in `Dockerfile` and `script/bootstrap`); `.golangci.yml` set to the
   org-standard v2-schema config used across the org's repos