chore: remove stale scaffolding, dead config, and BMAD leftovers (#57)

* chore: remove stale scaffolding, dead config, and BMAD leftovers

- Untrack the two BMAD skill files left behind under .claude/ (missed by 70483d7)
- Remove mkdocs.yml and docs/assets/: mkdocs is wired to no build step and its nav references seven files pruned in #52
- Remove docs/.archive/ (internal debug note + project scan report) from the public repo
- Remove empty features/demo scaffolding and redundant .gitkeep placeholders
- Drop dead Berkeley Mono @font-face declarations: the woff2 files were never shipped, every visitor 404s then falls back to JetBrains Mono
- Prune .gitignore entries for tooling that no longer exists (site/, _bmad, redisign scratchpads)
- Update CLAUDE.md and docs to match

* docs: note recoverable stats tests lost in #42 squash

Author: mariuspruvot