docs: remove obsolete planning artifacts

[abrichr]

Summary

• remove stale January planning, AI-generated summary, publication, and internal strategy documents from the public launcher repository
• update the remaining historical design index and architecture footer to route readers to canonical current documentation
• align CLAUDE.md and CONTRIBUTING.md with the launcher/engine boundary and the current local-first installation path

The current acquisition-first README and package metadata are intentionally unchanged; they already present pip install openadapt, openadapt quickstart, optional substrate extras, and the canonical openadapt-flow engine clearly.

Verification

• uv run --locked --extra dev pytest -q tests (58 passed, 6 skipped)
• uv run --locked --extra dev ruff check openadapt tests
• uv run --locked --extra dev ruff format --check openadapt tests
• built wheel and sdist with the pinned backend
• scripts/verify_release_artifacts.py passed
• inspected exact wheel/sdist contents: removed docs are outside the public package artifact and no workstation state is present
• noncanonical docs guard checks passed

No release is requested or authorized.