docs(plan): refresh or supersede stale implementation plan

[hansjm10]

Sequence

Order: 5 of 12
Phase: Phase 1 - Navigation and Freshness
Design: docs/agent-first-workflow-design.md
Tracker: #908
Depends on: #912

Agent Role

Docs Agent

Scope

• Resolve stale claims in docs/implementation-plan.md that contradict the current repository state.
• Either refresh the current-state/roadmap sections or mark the plan superseded with links to current roadmap/design docs.
• Ensure agents no longer read outdated claims about missing CI, tests, content compiler, or persistence as current truth.

Context Packet

• docs/implementation-plan.md
• docs/agent-first-workflow-design.md
• docs/index.md
• docs/role-index.md
• package.json
• .github/workflows/ci.yml
• packages/core/README.md
• docs/content-compiler-design.md
• docs/content-schema-reference.md

Acceptance Criteria

• docs/implementation-plan.md no longer presents obsolete current-state claims as true.
• If superseded, the doc clearly identifies replacement docs and keeps historical context separate from active guidance.
• If refreshed, the current state references actual packages, commands, and CI behavior.
• Any sidebar/index references remain accurate.
• The change avoids unrelated roadmap expansion.

Validation

• pnpm --filter @idle-engine/docs test:ci
• pnpm docs:build

Notes

• This is a docs truth-maintenance issue, not a feature planning rewrite.

Refs #906