docs: remove all plan files

Plans are scratchpads for implementation; once code lands the spec
+ tests + commit messages carry the load. Remove the plans dir
entirely (including the pre-existing ones from prior work:
harmont-cli-examples-passing, publishable-harmont-cli,
readme-quickstart-rewrite, zig-js-parallel-demo).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: markovejnovic