docs: remove the planning directory

[retr0h]

Implements specify-documentation-homes tasks 4.1 and 4.2.

Removes docs/plans/ — 70 documents. Design records belong in the corpus as changes, where a requirement and its reasoning stay attached to each other; a planning document kept beside the code goes stale the moment the code moves past it, with nothing to catch the drift.

Reviewed before removal

Task 4.1 asked for a review first. Six carried decision rationale rather than step-by-step plans:

• 2026-03-09-unified-domain-endpoint-architecture-design.md — removing POST /job in favour of domain endpoints
• 2026-03-11-container-runtime-design.md — the pluggable runtime driver interface
• 2026-03-13-container-runtime-rename-design.md
• 2026-03-03-agent-facts-design.md
• 2026-03-06-file-deploy-template-design.md
• 2026-02-27-node-resource-api-restructuring.md

Each describes architecture that is already built and already documented in docs/docs/sidebar/architecture/. Nothing needed lifting into a change, and git history retains all seventy regardless.

Side effect worth noting

docs/plans/ held the last stale references in the repository — 16 broken relative links and 10 mentions of github.com/golang/mock, which correct-documentation-drift deliberately left in place because this change removes the directory wholesale. Both counts are now zero.

Verification

• just docusaurus-build succeeds — the site's content root is docs/docs/, so docs/plans/ was formatted but never built
• just docusaurus-fmt-check and just md-fmt-check pass

Task 4.4 — noting in CONTRIBUTING.md that design records are changes — cannot be done here yet: osapi has no root CONTRIBUTING.md until standardize-repository-layout task 4.5 creates it.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@           Coverage Diff           @@
##             main     #448   +/-   ##
=======================================
  Coverage   99.94%   99.94%           
=======================================
  Files         484      484           
  Lines       24935    24935           
=======================================
  Hits        24921    24921           
  Misses         14       14           

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