chore(misc): remove 6 sub-project / misc rot files

  - echidna-playground/ROADMAP.adoc      5-line stub superseded by the
                                         494-line sibling roadmap.adoc
  - echidna-playground/ABI-FFI-README.md unfilled RSR template ({{PROJECT}}
                                         placeholder never replaced)
  - echidna-playground/CONTRIBUTING.md   recommends Nix fallback, contra
                                         estate ruling 2026-05-18;
                                         CONTRIBUTING.adoc is canonical
  - echidna-playground/LICENSE.txt       byte-identical duplicate of
                                         LICENSE; REUSE prefers the
                                         unsuffixed name + LICENSES/
  - tests/SANITY_SUITE_README.md         one-line note for a suite removed
                                         in 2026-04-17 — note no longer
                                         serves a purpose
  - NPM_BANNED.md                        policy duplicated verbatim in
                                         CLAUDE.md "BANNED" table; one
                                         canonical home is enough

https://claude.ai/code/session_01YPqu7gti4azBach6ZvpRFJ

Author: claude