chore(website): remove the cockpit retirement spec (#1085)

The cockpit → unified-docs migration is finished: no cockpit/**/docs/guide.md
walkthroughs remain, no capability descriptor declares docsAssetPaths, and
cockpit.threadplane.ai appears nowhere in the website outside specs asserting
its own absence.

This deliberately gives up two guards that were still live, so recording what
goes unenforced from here:

- cockpit.threadplane.ai may reappear in website links and authored content.
- "Cockpit", the retired product name, may reappear in user-facing .mdx/.md/
  .tsx. The spec had been arbitrating this across a dozen content files,
  allowing legitimate technical tokens — repo paths like cockpit/ag-ui/
  interrupts, package names like @threadplane/cockpit-registry, Nx targets
  like serve:cockpit — while barring the bare product name. The cockpit/
  workspace is still very much alive, so the source of that leakage remains.

public-copy.spec.ts does not backstop either one; it covers barred absolute
claims, telemetry positioning and retired docs routes.

Also retires the cwd bug that made this file fail whenever vitest ran from
anywhere but the repo root: it resolved its roots with join(process.cwd(),
'apps/website') instead of the sibling convention resolve(__dirname, '../..'),
so a targeted run reported five red herrings.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: blove