docs: prune private-era planning notes

[saagpatel]

What

• Remove private-era roadmap, sprint, handoff, decision, and agent execution notes from the public docs tree.
• Add docs/project-history.md as a concise public history page and keep docs/plans/README.md as a redirect-style archive note.
• Move the still-useful approval packet checklist into docs/writeback-safety-model.md and update doc-contract tests to use active docs.

Why

The repo is public now. The old planning files were not secret-bearing, but they exposed local-path and worker-instruction noise that made the project look like an internal operating log instead of a maintained public tool.

How

• Kept current product docs authoritative: README, modes, architecture, release gates, weekly review, and writeback safety.
• Pruned detailed historical plans from docs/plans/ and docs/superpowers/.
• Preserved the project-level lessons in a short public history page.

Testing

• python3 -m pytest -q -p no:cacheprovider - 2066 passed, 2 skipped, 1 warning.
• ruff check src/ tests/ - passed.
• Markdown local link check - passed.
• gitleaks detect --source . --redact --verbose - no leaks found.

Performance Impact

None expected; docs and doc-contract tests only.

Risk / Notes

• This intentionally removes old detailed planning files from the current public tree. They remain in git history, but no longer appear as active documentation.
• Local git push is blocked by app policy in this environment, so the branch commit was created through the GitHub API.