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.

tasks.md

@@ -273,4 +273,3 @@ Sizes: **S** = under a day, **M** = 1–3 days, **L** = 3–7 days.
 - [ ] **X-03** Periodic dependency updates (`dependabot` or `renovate`)
 - [ ] **X-04** Threat-model review at end of each phase
 - [ ] **X-05** Proper first-run onboarding UI: admin shouldn't need to `curl` `/api/bootstrap` by hand. Render the bootstrap form on the same login page (extra "setup token" field shown only while no admin user exists, hidden after); on submit POST to `/api/bootstrap`, then drop straight into a session. Surface the one-time token from the server log somewhere copy-able (or print a clickable URL with the token in the query string at first-run). Also: relax the 12-char password floor for the first-run path or document it in the form so `admin` doesn't silently fail validation.
-- [ ] **X-06** Lint-baseline cleanup pass. `.golangci.yml` is now on the v2 schema; CI is gated with `only-new-issues: true` because the repo carries ~90 pre-existing findings (gofumpt drift × 31, misspell × 25, missing godoc on exported consts × 10, bodyclose × 6, errcheck × 12, errorlint/nilerr/unused × handful) accumulated before lint was actually wired into CI. Drive the count to zero in a dedicated PR (mostly mechanical: `gofumpt -w .`, fix typos, add comments, audit nilerr cases since those *might* be real bugs), then drop `only-new-issues: true` so future regressions are caught at the source.