Remove the draft-PR preview skip and the preview label

Draft PRs currently skip preview deploy + e2e unless they wear the preview label, are marked ready, or a human dispatches with --allow-draft. Misha: "get rid of the draft-doesnt-get-preview thing it's too annoying" — and remove the preview label logic and everything downstream of it.

Rationale: the policy dated from a nine-slot fleet that a busy bedtime of agent-opened drafts could exhaust. The fleet is nineteen slots now, and the policy costs more in confusion (PRs silently without previews) than it saves in capacity. If contention returns, tasks/stealable-preview-leases.md is the finer-grained fix.

Checklist#

• remove decideDraftPreviewPolicy, the skip/teardown branches, previewOptInLabel, the draft PR-body notice, and --allow-draft from scripts/preview/preview.ts — deploy now always claims a slot; dispatch keeps --all-apps only
• drop draft/label lifecycle triggers (ready_for_review, converted_to_draft, labeled, unlabeled) and the label-filter job if: from .depot/workflows/cloudflare-previews.yml — opened/reopened/synchronize suffice
• simplify preview status diagnosis: no more "preview-eligible" split — previewEligibleWithoutSlotCount → openWithoutSlotCount; open-PR listing no longer fetches draft/label state
• update tests — deleted the policy suite; kept a dispatch --all-apps assertion; diagnosis fixtures simplified
• docs: CLAUDE.md, README.md, docs/pull-requests.md, docs/dev-environments.md, docs/adding-preview-slots.md — canary runbook no longer labels drafts
• mark tasks/stealable-preview-leases.md as premise-removed — status: maybe, only relevant if slot contention returns

Implementation notes#

• The workflow's on: triggers are registered from the default branch, so drafts start getting previews once this merges. The preview script runs from the PR head though, so this PR itself deploys a preview despite being a draft — self-demonstrating.
• Existing preview-labeled PRs are unaffected; the label just becomes inert.
• pr-dashboard.yml keeps its converted_to_draft trigger — that's for the dashboard, unrelated to the preview policy.