chore: remove cargo-workspaces GitHub Actions workflow (#239)

The cargo-workspaces workflow consistently failed due to branch protection
rules and GitHub's restrictions on bypassing them.

Replaced with a reliable manual release script (scripts/release.sh) that:
- Uses local credentials (no token complexity)
- Provides interactive confirmations
- Has better error handling and recovery
- Bypasses all GitHub Actions permission issues

Keeping essential workflows:
- ci.yml (tests/lint)
- release.yml (cargo-dist for binaries)
- docker.yml (Docker images)
- security.yml (security audits)
- cargo-deny.yml (dependency checks)
- mdbook.yml (documentation)

Author: joshrotenberg