ci: update dependabot-changeset workflow to install deps, update lockfile, and support workflow_dispatch

[phoenix-server]

Changes

• Adds workflow_dispatch trigger with inputs for pr_number, pr_title, and head_ref so the workflow can be run manually against existing open dependabot PRs
• Installs pnpm dependencies after checkout (pnpm 10.33.0, Node 24) so the lockfile gets regenerated
• Commits pnpm-lock.yaml if it changed after install, alongside the changeset file
• Merges both the lockfile update and changeset into a single commit to keep the PR history clean

[changeset-bot]

⚠️ No Changeset found

Latest commit: cb071de

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

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