Remove duplicated kit demos/docs + heavy feature envs from rclcppyy (M3 cleanup)

The kits, their demos, docs, and dev envs are canonical in the cppyy_kit repo now;
rclcppyy is the drop-in accelerator product. Remove the duplicates:

- Kit demo scripts: scripts/{bt_kit,control_kit,moveit_kit,nav2_kit,ompl_kit,
  pcl_kit}_demos/, scripts/{tf_demos,vision,freeze,datasets}/.
- Kit docs: docs/{bt_kit,control_kit,moveit_kit,nav2_kit,ompl_kit,pcl_kit,tf,vision}/,
  docs/tutorials/, docs/kits/{COMMON_PATTERNS,FREEZE}.md. Kept docs/kits/
  ARCHITECTURE_V2.md (the historical proposal) with a header pointing at the suite
  repo as the living home.
- pixi.toml: drop the bt/pcl/ompl/nav2/moveit/control/vision/cudabuild/vision-cuda
  feature envs + tasks, and the tf-demo tasks in the default [tasks] block. Re-lock:
  pixi.lock 22886 -> 11926 lines (envs: default, demos, pkg).

Kept: the product core (rclcppyy/, test/), scripts/{benchmarks,ros_tutorials,
big_messages_demos}, the demos env (serves the kept big_messages + roscon demos),
roscon_uk_2025 archive, recipe/, workflows, RELEASING.md, the pre-release PYTHONPATH
bridge (workspace_activation.sh). PLAN.md untouched.

Gates (ROS_DOMAIN_ID=57): build 0; lint 0; test 26 passed (core 16 + 10 kit-shim
smokes); bench 3000 msgs, 0 dropped (parity). No test/ or workflow referenced a
removed path.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: awesomebytes