lint: drive baseline to zero, drop only-new-issues gate

Cleanup pass over the repo so CI can enforce lint going forward
without the only-new-issues escape hatch:

* gofumpt -w across the tree (31 hits, all formatting)
* misspell --fix (25 hits, US-locale spelling) — but reverted on
  api.JobCancelled = "cancelled" since that literal is the wire +
  DB CHECK constraint value, plus matched the case in store/fleet.go
  back to "cancelled" and added //nolint:misspell on both for the
  next time someone reaches for the auto-fix
* Wrap every `defer rows.Close()` / `defer stmt.Close()` /
  `defer res.Body.Close()` in `defer func() { _ = .Close() }()`
  to satisfy errcheck without losing the close itself
* websocket.Dial callers (1 prod, 4 tests) now capture + close the
  upgrade response Body — coder/websocket can return res with a nil
  Body on success, so the test deferred-closes guard against that
* Annotate the two genuine-by-design nilerr cases with //nolint
  comments explaining why nil-on-error is the contract (cookie
  missing = no session; ctx cancelled mid-backoff = clean shutdown)
* Add brief godoc on the 10 exported const groups + types that
  revive flagged (api.HostOS/HostArch/JobKind/JobStatus/LogStream/
  ErrorCode, restic.EventKind, store.Role, web.FS)
* Drop the unused (*Server).userByID method
* Inline the unparam baseView(active) — every UI page is under
  the dashboard primary nav today

Result: `golangci-lint run ./...` reports 0 issues. CI lint job
no longer needs only-new-issues: true; X-06 follow-up entry in
tasks.md removed.

internal/agent/sysinfo/sysinfo.go

@@ -48,7 +48,9 @@ func Collect(ctx context.Context, resticPath string) (Snapshot, error) {
 
 // detectResticVersion runs `restic version` and parses the first line.
 // Output looks like:
-//   restic 0.17.1 compiled with go1.22.5 on linux/amd64
+//
+//	restic 0.17.1 compiled with go1.22.5 on linux/amd64
+//
 // Returns the version token (e.g. "0.17.1") or "" if restic isn't
 // found. We never block startup on a missing restic — the operator
 // might not have installed it yet, and the agent should still be
@@ -74,5 +76,5 @@ func detectResticVersion(ctx context.Context, override string) (string, error) {
 	if len(parts) >= 2 && parts[0] == "restic" {
 		return parts[1], nil
 	}
-	return "", fmt.Errorf("sysinfo: unrecognised restic version output: %q", first)
+	return "", fmt.Errorf("sysinfo: unrecognized restic version output: %q", first)
 }