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/wsclient/client.go

@@ -40,7 +40,7 @@ type Config struct {
 // Sender is what handlers use to push agent → server messages
 // (job.progress, job.finished, log.stream, command.result, …).
 // Returned by the WS client to the dispatch handler. Write operations
-// serialise behind a single mutex on the conn; concurrent calls are
+// serialize behind a single mutex on the conn; concurrent calls are
 // safe.
 type Sender interface {
 	Send(env api.Envelope) error
@@ -52,7 +52,7 @@ type Sender interface {
 type Handler func(ctx context.Context, env api.Envelope, tx Sender) error
 
 // Run keeps the agent connected indefinitely. Returns when ctx is
-// cancelled. Errors during a single connection attempt are logged and
+// canceled. Errors during a single connection attempt are logged and
 // trigger reconnect-with-backoff; only ctx.Done() ends the loop.
 func Run(ctx context.Context, cfg Config, handle Handler) error {
 	if cfg.HeartbeatPeriod <= 0 {
@@ -69,7 +69,10 @@ func Run(ctx context.Context, cfg Config, handle Handler) error {
 			slog.Warn("ws agent disconnect", "err", err)
 		}
 		if err := sleepCtx(ctx, backoff.next()); err != nil {
-			return nil
+			// ctx cancellation mid-backoff means the parent shut us down —
+			// exit the reconnect loop quietly rather than propagating
+			// a context error up to a caller that will discard it.
+			return nil //nolint:nilerr
 		}
 	}
 }
@@ -100,11 +103,15 @@ func connectOnce(ctx context.Context, cfg Config, handle Handler) error {
 	}
 
 	dialCtx, cancel := context.WithTimeout(ctx, 30*time.Second)
-	conn, _, err := websocket.Dial(dialCtx, wsURL, dialOpts)
+	conn, res, err := websocket.Dial(dialCtx, wsURL, dialOpts)
 	cancel()
 	if err != nil {
 		return fmt.Errorf("dial: %w", err)
 	}
+	// websocket.Dial returns the upgrade response separately from the
+	// conn. Body is empty on a successful upgrade but Go's net/http
+	// still expects it closed to release the connection.
+	defer func() { _ = res.Body.Close() }()
 	defer conn.CloseNow() //nolint:errcheck
 
 	// Send hello.

internal/agent/wsclient/enroll.go

@@ -50,7 +50,7 @@ func Enroll(ctx context.Context, serverURL string, req EnrollRequest) (*EnrollRe
 	if err != nil {
 		return nil, fmt.Errorf("agent enroll: post: %w", err)
 	}
-	defer res.Body.Close()
+	defer func() { _ = res.Body.Close() }()
 	rawRes, _ := io.ReadAll(res.Body)
 	if res.StatusCode != stdhttp.StatusCreated {
 		return nil, fmt.Errorf("agent enroll: server returned %d: %s",