chore: stop syncing to 365-skills, keep this repo standalone

The skill is distributed from this repository alone now: the workflow that
rsynced skills/drawio-skill/ into Agents365-ai/365-skills and bumped its
marketplace entry is removed, and the docs no longer advertise the 365-skills
plugin marketplace.

- delete .github/workflows/sync-365-skills.yml and its path filter in tests.yml
- README / README_CN / INSTALL_SKILL(_CN) / Pages pages: drop the Claude Code
  Plugin badge, the /plugin marketplace add + /plugin install drawio block and
  the /plugin update drawio line
- the any-agent path becomes `npx skills add Agents365-ai/drawio-skill -g`
  (verified against the skills CLI), manual clone instructions are unchanged