feat(plugins): add agent-assisted setup

[JedIV]

Summary

• add an explicit setup skill that checks uv first, offers the official platform installer with explicit approval, and verifies the Dataiku connection
• make Set up Dataiku Headless the primary Codex starter prompt
• route first-install and MCP recovery from the main Dataiku skill into the setup workflow
• update the README so users can install the plugin before handling the runtime prerequisite

This complements #91, which adds portable Agent Plugins packaging for Cursor and other conforming clients.

Test plan

• plugin-creator validation
• skill validation for setup and dataiku-headless
• claude plugin validate .
• uv run pre-commit run --all-files --show-diff-on-failure
• uv run pytest (235 passed)
• uv build
• uv run cz check --rev-range origin/main..HEAD

[pmasiphelps]

changed the name of the new skill from setup -> dataiku-headless-setup so that it wont conflict with other skills. looks good!