chore: remove implemented plan docs from .opencode/plans

All three plans are fully implemented and committed; verified against the
codebase before removal:
- fix-control-plane-tls-1766.md          -> d2b53bf
- fix-verify-false-positive-1757.md      -> 9811d05
- python-ruby-release-infrastructure.md  -> 3662339, 4aecaae, 71f7b57

Author: jamesdbloom