ci(docs): smoke start, troubleshoot, reference, and integration journeys

[CompleteDotTech]

Summary

Completes the browser-smoke coverage required by issue #778 for the four
canonical journey routes: start, troubleshoot, reference, and
integration.

• The coven-docs pull-request CI lane itself already exists: docs.yml runs on
  every pull_request and executes pnpm verify (build + content/link/anchor
  validation + browser smoke). It landed in refactor: certify and redesign the Coven documentation release surface #56, after #778 was filed — this PR
  does not duplicate it.
• The existing smoke, however, covered no integration route at all. This
  change tags the existing start/reference/troubleshoot journey stops and adds
  the missing integration stops: /docs/harnesses and /docs/harnesses/codex
  (the Harnesses section is Coven's integration surface: built-in harness
  adapters — codex, claude, coven-code, copilot).
• scripts/smoke-docs.mjs now hard-fails if any required journey loses its
  routes, and records the journey → route map in output/docs-smoke/report.json
  as CI evidence (uploaded by docs.yml as docs-certification-* artifacts).

Slice note: this is the remaining slice of #778's "browser smoke for start,
troubleshoot, reference, and integration routes"; the PR CI (build + link
validation) part of #778 was already delivered by #56 and is unchanged here.

Source of truth

• ci(docs): add canonical docs build, link, and browser journey coven#778 and umbrella docs: progressive disclosure, single-source docs, and E2E certification coven#670 ("coven-docs PR CI
  builds, validates links, and browser-tests the primary journey").
• Expected text verified against the owning content:
  content/docs/harnesses/index.mdx ("Built-in harnesses") and
  content/docs/harnesses/codex.mdx ("Coven launches it as a project-rooted
  PTY").
• Section stability from docs/site-manifest.json (harnesses: stable, owner
  OpenCoven/coven), rendered through components/docs-status.tsx.

Issue

Refs OpenCoven/coven#778
Closes OpenCoven/coven#778

Stability impact

• No product-contract change

Public surface

• No public-surface change

CI smoke coverage only; no routes, redirects, navigation, or visuals changed.

Test plan

• pnpm typecheck
• pnpm check (content, site-manifest, link, anchor, automation checks +
  23 api-runner tests) — all green
• pnpm build:site from the working tree — build green (86 doc paths)
• SSR assertion pass against the built site for all five journey routes:
  HTTP 200, expected text, exactly one h1/main, canonical link,
  data-docs-stability="stable", contract-source link
• git diff --check
• Full browser smoke pnpm verify (Chromium leg) — deferred to CI; this
  sandbox cannot run Chromium (no system X/Glib libraries, no root)

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