Implementation report

Summary

Reorganized maintained repository documentation around current Yoi design intent and development workflow.

Replaced the root README.md with a concise developer entry point.
Added docs/README.md as the documentation map and contribution boundary.
Added maintained design docs under docs/design/:
- overview.md
- pod-session-state.md
- context-history.md
- profiles-manifests-prompts.md
- tool-permissions-scope.md
- memory-knowledge.md
- compaction.md
- provider-model-boundary.md
Added maintained development docs under docs/development/:
- work-items.md
- workflows.md
- validation.md
- dogfooding.md
- environment.md
Moved old top-level docs, docs/plan/, docs/ref/, and docs/research/ material into ignored docs/.local/old-docs/ so the repository documentation surface no longer presents it as current authority.

./tickets.sh doctor => doctor: ok
git diff --check => passed
stale-surface grep over README.md, maintained docs/, and crate READMEs excluding docs/report/ and docs/.local/ => no output