chore: remove inherited issue auto-close gate and pi leftovers in .github (#152)

* chore: remove inherited issue/PR auto-close gate

The contributor gate workflows inherited from pi auto-closed every issue
and PR opened by non-collaborators. Remove the gate workflows, the
approved contributors list, and the auto-close wording in the issue
templates, CONTRIBUTING.md and the coding-agent README.

* chore: remove remaining pi leftovers from .github

- Point issue template CONTRIBUTING links to stepfun-ai/Step-Code
- Use `step -ne` and the current version in the bug template
- Drop the new-contributor requirement from the proposal template
- Remove pi's Discord contact link and blank-issue restriction
- Remove pi's package-report template and inprogress label workflow

Author: kylin1020