chore(lint): add golangci-lint config and fix the 71 findings it surfaces

[gregggreg]

What

Adds a curated .golangci.yml and fixes the 71 issues it surfaces. The repo previously had no lint config, so golangci-lint run used defaults only and reported 0 issues — a lint pass without a config would have delivered nothing.

Linters enabled on top of the standard set (errcheck, govet, ineffassign, staticcheck, unused): errorlint, nilerr, gocritic, revive, unconvert, misspell, bodyclose.

Changes, in order of value

Correctness — 9 errorlint findings. These are latent bugs that surface as error wrapping spreads through the codebase:

• err != X → errors.Is in cmd/nightshift/commands/daemon.go, cmd/provider-calibration/main.go (2 sites)
• error type assertions → errors.As for *exec.ExitError in internal/agents/{claude,codex,copilot}.go
• %v/%s → %w in three fmt.Errorf calls in internal/scheduler/scheduler.go

8 nilerr findings — annotated, not rewritten. Every site (internal/budget/budget.go ×2, internal/integrations/{github,td}.go ×4, internal/providers/claude.go ×2) is deliberate graceful degradation: a missing integration or an unreadable optional file falls back to a default rather than failing the run. Changing that control flow would alter runtime behavior under the guise of a style pass, so each carries a //nolint:nilerr with a one-line justification instead. None turned out to be a genuinely dropped error.

6 gocritic findings. Three if-else chains → switch (daemon.go ×2, preview_output.go); three Deprecated: notices moved into their own paragraph.

48 revive findings. Package/const-block doc comments, malformed comment forms, a single canonical // Package providers comment (the per-file comments in claude.go/codex.go/copilot.go no longer read as package comments), a justification on the blank modernc.org/sqlite import, min/max/real builtin shadows renamed to context-appropriate locals and named results, and unused parameters renamed to _.

CI. .golangci.yml uses the golangci-lint v2 config schema, which golangci-lint-action@v6 (a v1 driver) cannot parse — the Lint job would have broken the moment this merged. Bumped to @v8 pinned at v2.12.2, and raised CI's Go version from 1.23 to 1.24 to match the go directive in go.mod.

Deliberate exclusions

• internal/stats.StatsResult (revive stutter warning) is left unrenamed. It's an exported cross-package API rename and does not belong in a lint PR. Excluded in .golangci.yml with an in-config justification.
• unused-parameter in _test.go files. Mock implementations must match interface signatures; renaming those parameters to _ costs readability for no benefit.
• website/ (Docusaurus + node_modules) is untouched — no lint config exists there and it's outside the Go module.

Verification

Run on this branch, rebased onto main, after golangci-lint cache clean:

$ gofmt -l .
(no output)

$ go vet ./...
(no output)

$ go build ./...
(no output)

$ golangci-lint run ./...
0 issues.

$ go test ./...
21 packages ok, 0 failures

The test suite was recorded as a baseline before any edit and re-run after; results are unchanged, so no "style" fix altered behavior.

Note on history

Earlier iterations of this branch were stacked on docs-backfill. It has been rebased onto main, so the PR now carries only the 7 lint commits with no unrelated docs churn.