chore(repo): prune process artifacts + reference docs to a lean known-good baseline

Remove 66 repo-only files (NONE shipped in the npm package per package.json files):
dogfood run-logs, docs/audit reports, docs/architecture diagrams + design spec,
docs/plans, .atlas-dogfood benches, .i-walkthroughs. Keeps docs/product/
(MODEL-ECONOMY.md is read by test_economy_report). Removed files were only cited in
test docstrings, never read — full offline suite stays green (883 passed, 5 skipped).
Architecture diagrams remain recoverable from git history.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: claude