ci: remove unsupported filter_unconventional release-plz key

[takaebato]

Problem

PR #42 added filter_unconventional = false to release-plz.toml, but that key is not supported by release-plz 0.3.159 (the latest release — it only exists in unreleased release-plz). Since #42 merged, the release-plz workflow on master now fails to parse the config:

unknown field `filter_unconventional`, expected one of `header`, `body`, `trim`,
`commit_preprocessors`, `postprocessors`, `sort_commits`, `link_parsers`,
`commit_parsers`, `protect_breaking_commits`, `tag_pattern`

Fix

Remove the unsupported key, restoring a valid config so release-plz works again.

Follow-up (separate)

The original goal — including pre-convention (non-type:) commits like #31/#33 and the external #7/#9 in the changelog — has no released config/upgrade fix (the toggle is unreleased). It is a one-time historical-backlog gap; future commits are all conventional (PR-title lint). We'll handle that gap manually rather than via config.

🤖 Generated with Claude Code

[github-actions]

✅ PR title follows the Conventional Commits spec.