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/scheduler/scheduler.go

@@ -110,7 +110,7 @@ func (s *Scheduler) Apply(payload api.ScheduleSetPayload, tx Sender) {
 		"received", len(payload.Schedules), "active", added)
 
 	// Ack outside the lock — Send() shouldn't take long, but holding
-	// s.mu across an external call would needlessly serialise other
+	// s.mu across an external call would needlessly serialize other
 	// callers (e.g. a future Status() inspection from the UI).
 	ackEnv, err := api.Marshal(api.MsgScheduleAck, "", api.ScheduleAckPayload{
 		Version:   payload.Version,
@@ -167,4 +167,3 @@ func (s *Scheduler) fire(entry api.Schedule) {
 			"schedule_id", entry.ID, "err", err)
 	}
 }
-