ci: automate release flow with PR-based publishing

[felippemsc]

Summary

• Add release-pr.yml workflow: automatically creates a version bump PR when a GitHub Release is published
• Rework release.yml triggers: publish to PyPI and crates.io only when a release PR is merged (detected via version change in Cargo.toml)
• Remove pull_request trigger from release workflow to avoid unnecessary wheel builds on every PR

Release flow

1. Create a GitHub Release with tag v0.2.0
2. release-pr.yml creates branch release/0.2.0, bumps version in Cargo.toml + pyproject.toml, opens PR
3. Review and merge the PR
4. release.yml detects version change → publishes to PyPI + crates.io automatically

What runs when

Event	CI	Wheel builds	Publish
PR to main	Yes	No	No
Push to main (regular)	Yes	Yes	TestPyPI only
Push to main (release merge)	Yes	Yes	PyPI + crates.io + TestPyPI

Setup required

• Add CARGO_REGISTRY_TOKEN secret to the repo (from crates.io API Tokens)

Test plan

• Verify CI still runs on PRs (no wheel builds)
• Verify wheel builds run on push to main
• Test release flow with a GitHub Release