Stack context: two-layer stack tightening Go lint gates. This bottom
layer fixes the lint exclusion; the layer above adds new rules.
Why: a `.git` path exclusion in the golangci config was hiding Go files,
masking 70 findings. This fixes the exclusion, fixes the findings
(`agent/agentgit` now runs git through `agentexec.Execer`), and adds a
`make lint/go` check that fails when an exclusion hides Go files. `make
lint/go` now waits for a concurrent golangci-lint run instead of
failing. The exhaustruct exclusion rule for `scripts/*` had the same
problem: as a regex it also matched `agent/agentscripts` and
`coderd/agentapi/scripts.go`, so it is now `^scripts/`.
Validation: full-tree lint reports 0 issues; the new check fails on the
old entry and passes on the new one.
> Xum acted for Mike to open and update this PR.
Michael Suchacz
e8dcf9a3f1