chore: use semantic PR title for flake update workflow

[stackptr]

Summary

• Changes the auto-generated flake update PR title from Update flake inputs to chore: update flake inputs to follow the conventional commit style used in this repo
• Replaces the static Automatic update of flake inputs body with the commit message generated by nix flake update --commit-lock-file, which includes a structured breakdown of exactly which inputs changed and what revisions they moved between

Test plan

• Trigger the workflow manually via workflow_dispatch and verify the created PR has title chore: update flake inputs
• Verify the PR body shows the flake input diff from the lock file update commit

[stackptr]

• chore: use semantic PR title for flake update workflow #453 👈 (View in Graphite)
• main

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