Post-rebase fixup for the docs split (#93). The plan doc was added
to docs/ at the top level before main reorganized to docs/{user,dev}/.
This moves it into docs/dev/ and adds an entry to docs/dev/index.md
under a new "Active Implementation Plans" section so the
check-agents-md.sh link check passes.
Per the original commit message (617a77d), the plan doc is intentionally
temporary — it will be deleted when v1 lands.
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Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
