fix: dispatch release PR CI

[haasonsaas]

Summary

• allow ci.yml to be triggered with workflow_dispatch
• let the release-please workflow dispatch CI for generated release PR branches when it falls back to GITHUB_TOKEN
• document why this keeps Release Please PRs from getting stuck without normal pull_request events

Test plan

• actionlint
• python3 scripts/validate_release_metadata.py
• python3 scripts/validate_pr_title.py 'fix: dispatch release PR CI'
• git diff --check

[cursor]

PR Summary

Low Risk
Low risk: changes are limited to GitHub Actions workflow triggers/permissions and a new dispatch step; main risk is misconfigured permissions or branch parsing causing CI not to run for release PRs.

Overview
Release Please PRs now reliably get CI even when the workflow uses GITHUB_TOKEN (which may not emit follow-on pull_request events).

ci.yml is updated to support manual workflow_dispatch, and release-please.yml gains actions: write plus a step that parses Release Please’s created PR branches and runs gh workflow run ci.yml --ref <branch> for each. Documentation is updated to explain the GITHUB_TOKEN limitation and why the explicit dispatch is needed.

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