Reduce unnecessary GitHub Actions usage

[andy5090]

What changed

• run normal CI only for pull requests or manual dispatch, not again after merge to main
• cancel superseded CI runs for the same pull request
• remove the duplicate i686 build from PR CI; it remains part of version-tag releases
• set cargo-dist pr-run-mode = "skip" and remove pull-request triggers from the generated release workflow
• document the reduced Actions policy for releases

Why

Every change previously triggered normal CI and i686 builds both on the pull request and again after merging, while cargo-dist also ran a release plan on each pull request. This consumed GitHub Actions minutes without adding distinct validation.

Validation

• parsed all workflow YAML files successfully
• verified CI has only pull_request and workflow_dispatch triggers
• verified Release has only version-tag push triggers
• verified cargo-dist configuration persists PR skipping
• git diff --check

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