Clean up generated changelog formatting
## Context
The `cliff.toml` configuration generates a changelog from conventional commits, but the current output has a few rough edges that hurt readability.
## Desired changes
- **Remove brackets around the version** — change `## [{{ version }}]` to `## {{ version }}` in the body template.
- **Add emojis to group titles** — prefix each group name with a relevant emoji (e.g. ✨ Features, 🐛 Bug Fixes, 🔒 Security, ⚡ Performance, 📚 Documentation).
- **Order groups by importance** — currently groups are alphabetised by `group_by`. Force the order: Features → Bug Fixes → Security → Performance → Documentation. This likely means prefixing group names with a sort key (e.g. `"<!-- 0 -->✨ Features"`) or using `commit_parsers` with `default_scope`/ordering tricks documented by git-cliff.
- **Link merge requests when possible** — append a link to the originating MR for each commit (e.g. `- feat: foo ([!42](…/merge_requests/42))`). git-cliff exposes `commit.remote.pr_number` when the GitLab integration is configured; alternatively, parse the MR number from the commit message footer (`See merge request !42`).
## Scope
- Update `cliff.toml`
- Regenerate `CHANGELOG.md` to validate the new output
- No changes to the release pipeline itself
## References
- Current config: `cliff.toml`
- git-cliff docs: https://git-cliff.org/docs/configuration
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