chore: prepare repository for open-source release

Migrate CI/CD from self-hosted ARC runners to GitHub-hosted runners:
- Replace arc-runner-codex with ubuntu-latest/ubuntu-22.04
- Remove container blocks, apt-get installs, and manual Rust setup
- Replace PVC cache with upload/download-artifact
- Re-enable build provenance attestation and npm --provenance
- Restore full cross-platform dist targets in Cargo.toml
- Remove leftover Gitea workflow files

Update documentation for public repository:
- Add active development notice to README
- Replace userjot.com feature board links with GitHub Issues
- Fix broken config link in README
- Update doc site navbar and footer links
- Add GitHub issue templates for bug reports and feature requests

Author: AshDevFr