feat(examples): add PR bump preview example

[bearomorphism]

Description

Adds an example workflow examples/pr-bump-preview.yaml showing how to use setup-cz to post a sticky comment on every pull request previewing the version bump that would happen on merge. This addresses commitizen-tools/commitizen#1510.

What it does

• Triggers on pull_request_target so the workflow has pull-requests: write permission for fork PRs too. Only cz bump --dry-run (a read-only command) is executed against the PR commits.
• Uses commitizen-tools/setup-cz@main to install the cz CLI.
• Captures cz bump --dry-run --yes output and exit code:
  • 0: dry-run succeeded — the comment shows the next version, tag, increment, and changelog preview.
  • 21 (NoneIncrementExit): treated as "no eligible commits for a bump" instead of a workflow failure.
  • Any other code: surfaced inside the comment so reviewers can see what went wrong.
• Uses peter-evans/create-or-update-comment@v4 with a hidden <!-- commitizen-bump-preview --> marker so the comment is replaced (rather than duplicated) on every push.

Companion changes

• commitizen-tools/commitizen PR #1957 adds the same workflow to the commitizen repo and documents the pattern.
• commitizen-tools/commitizen-action PR #102 adopts the same pattern.

Closes commitizen-tools/commitizen#1510

Expected output

The workflow posts (and replaces on every push) a single sticky comment whose body depends on the dry-run exit code.

cz bump --dry-run --yes succeeds (status 0) — eligible bump:

Rendered comment

🔍 Commitizen bump preview

Merging this PR will produce the following bump:

bump: version 4.15.1 → 4.16.0
tag to create: v4.16.0
increment detected: MINOR

<!-- commitizen-bump-preview -->
## 🔍 Commitizen bump preview

Merging this PR will produce the following bump:

```
bump: version 4.15.1 → 4.16.0
tag to create: v4.16.0
increment detected: MINOR
```

NoneIncrementExit (status 21) — no eligible commits:

🔍 Commitizen bump preview

No commits in this PR are eligible for a version bump.

Any other non-zero status — error surfaced inside the comment:

🔍 Commitizen bump preview

⚠️ cz bump --dry-run exited with status 3:

NoCommitsFoundError

[bearomorphism]

Note for reviewers: the duplication across this repo, commitizen-action, and setup-cz is intentional for now. Tracked as a follow-up in commitizen-tools/commitizen#1959 (mirrored at #20) — once these PRs are merged and we have one or two real bump-preview comments in production, we plan to promote the example into a reusable workflow in setup-cz and shrink the workflows in commitizen + commitizen-action to ~8-line wrappers pinned to a tagged setup-cz release.