added --exclude-pr option to filter PRs by title

[siphonite]

What

Adds a new repeatable --exclude-pr <string> option to the changelog subcommand.

When provided, pull requests whose titles contain any of the specified strings
are excluded from the generated changelog.

Example:
magg changelog ... --exclude-pr "Release" --exclude-pr "Bump"

---

Why

Some repositories contain automated or release-related PRs that should not
appear in human-readable changelogs.

This feature allows maintainers to easily filter out such PRs, as requested in #2.

---

How

• Added a repeatable CLI flag to the changelog subcommand
• Wired the flag through the Action enum and call chain
• Filtered PRs before insertion into pull_request_map (per maintainer hint)
• Preserved existing behavior when the flag is not used

---

Notes

• No existing behavior is changed unless --exclude-pr is provided
• Filtering is based on simple substring matching, as requested
• Commits from excluded PRs remain listed as standalone commits

Closes #2

[DariuszDepta]

LGTM