Fix release flow + bump to v0.4.0

[colinrozzi]

Summary

• Bump version to 0.4.0 to match what's already published on crates.io
• Rework release flow: nix run .#release -- patch now creates a PR instead of tagging directly
• Publish workflow triggers on merge to main when version changes (not on tags)
• Workflow creates git tag + GitHub release after successful publish
• Fix CI to reference packr-abi (was pack-abi)

Release flow

nix run .#release -- patch    # bumps version, creates PR
                               # review + merge
                               # CI publishes to crates.io, creates tag + release

Test plan

• nix flake check passes
• Versions consistent across Cargo.toml and flake.nix

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