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/store/store_test.go

@@ -34,10 +34,12 @@ func TestOpenAppliesMigrations(t *testing.T) {
 	}
 
 	// Spot-check a few tables exist with expected columns.
-	tables := []string{"users", "sessions", "hosts", "repos",
+	tables := []string{
+		"users", "sessions", "hosts", "repos",
 		"credentials", "schedules", "jobs", "job_logs",
 		"snapshots", "alerts", "audit_log",
-		"enrollment_tokens", "host_schedule_version"}
+		"enrollment_tokens", "host_schedule_version",
+	}
 	for _, tbl := range tables {
 		row := s.DB().QueryRow(
 			`SELECT name FROM sqlite_master WHERE type='table' AND name = ?`, tbl)