Fix dead links and gate the docs build in CI

[prk-Jr]

Summary

• The VitePress build on main has been failing since Make the SSAT debug comment configurable #943 landed; Add shared root caching with hybrid ESI and streaming assembly #1013 added
  more breakage on top. Nineteen dead links across three files, now fixed, so
  the docs deploy goes green again.
• Dead links were only ever checked by deploy-docs.yml, which runs on push to
  main alone. Move the build into the existing format-docs job so the check
  runs on pull requests, before breakage reaches main.

Changes

File	Change
.github/workflows/format.yml	Run npm run build in the format-docs job. That job already runs on pull_request, already installs the docs dependencies, and is already a required status check, so dead links become a merge blocker with no ruleset change.
docs/superpowers/specs/2026-07-20-ssat-debug-comment-config-design.md	Twelve links pointed into the source tree via ../../../crates/.... Those targets sit outside the VitePress root and can never resolve. Rewritten as inline code spans, e.g. (`publisher.rs:950`), matching the 172 source references already written that way under docs/superpowers/.
docs/superpowers/archive/2026-08-08-esi-cacheable-root-validation-design.md	Two sibling links kept a ./ prefix after the document moved into archive/. Retargeted to ../specs/.
docs/superpowers/archive/2026-08-10-1009-esi-validation-spike.md	Five sibling links likewise retargeted to ../plans/. Every target already existed one directory over.

Closes

No linked issue — this repairs a build that is red on main right now.

Test plan

• Docs format: cd docs && npm run format — all matched files use Prettier code style
• Docs lint: cd docs && npm run lint — clean
• Docs build: cd docs && npm run build — build complete in 10.91s (was [vitepress] 19 dead link(s) found)
• cargo fmt --all -- --check
• Other: verified the new gate fails closed. A deliberate dead link exits 1 with [vitepress] 1 dead link(s) found; the build completes once removed.
• cargo test-fastly && cargo test-axum — not run, no Rust source changed
• cargo clippy-fastly && cargo clippy-axum — not run, no Rust source changed
• JS tests / JS format — not run, no JS source changed

Checklist

• Changes follow CLAUDE.md conventions
• No unwrap() in production code — no Rust changed
• Uses tracing macros (not println!) — no Rust changed
• New code has tests — the CI gate is itself the regression test, and it was verified to fail closed
• No secrets or credentials committed

Note on the two fix sets

The two halves have different origins, worth separating for anyone reading
git log later:

• The twelve source-tree links arrived with Make the SSAT debug comment configurable #943 and turned the docs deploy red
  at 04:34 on 2026-08-22.
• The seven archive links arrived with Add shared root caching with hybrid ESI and streaming assembly #1013, whose squash landed as
  d97bda69f. That commit carries the subject probe because of a tooling
  error during its merge; it is the "Add shared root caching with hybrid ESI and
  streaming assembly" merge, and its full message body is intact.