ci(release-please): add release notes at staging promotion

[amandazhuyilan]

Pull Request

Summary

• Wires up release-please to cut releases (version bump, CHANGELOG.md, GitHub Release, git tag) from the staging branch.
• release-please runs on push: [staging] with target-branch: staging, so a dev → staging promotion compiles the promoted commits into release notes.
• dev has no release line of its own — releases exist only at the promotion point (staging).
• No deployment impact: deploys are branch-driven (push to dev/staging); this only adds release notes/versioning on top.

How to use it

• Do feature work on dev: open PRs into dev and squash-merge them, so each feature becomes one clean commit.
• Promote with a MERGE COMMIT, not a squash: open a dev → staging PR and merge it using "Create a merge commit".
• After the promotion lands on staging, release-please opens/updates a release PR (release-please--branches--staging--…) previewing the version bump + notes.
• Merge that release PR to publish the GitHub Release + CHANGELOG.md + tag.

Activation (important — one-time)

• This workflow only fires once the file exists on the staging branch. So after merging this PR into dev, promote dev → staging to carry it across; the first release PR appears on that promotion.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added or updated documentation where necessary
• I have run linting and unit tests locally
• The code follows the project's style guidelines