chore: add changelog fragments

[piotr-iohk]

Closes #921

Description

This PR:

1. Adds changelog.d/next and changelog.d/hotfix fragment folders.
2. Adds release tooling to collect fragments into CHANGELOG.md and preview pending fragments in the terminal.
3. Updates agent, PR, and release guidance so normal PRs add fragments instead of editing CHANGELOG.md directly.

Preview

N/A

QA Notes

1. Collector no-op

• Run scripts/collect-changelog.sh --target next with no fragments and confirm it exits cleanly.
• Run scripts/collect-changelog.sh --target hotfix with no fragments and confirm it exits cleanly.

2. Preview pending fragments

• Add a temporary valid fragment under changelog.d/next/.
• Run scripts/preview-changelog.sh --target next and confirm it renders the entry grouped by category.
• Remove the temporary fragment.

3. Release collection

• Add a temporary valid fragment under changelog.d/next/.
• Run scripts/collect-changelog.sh --target next and confirm it is inserted under ## [Unreleased] and the fragment is deleted.
• Revert the temporary changelog change after testing.

Made with Cursor

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[ovitrif]

tAck

Besides the listed tests, I also tested some edge cases:

• 🟢 collect changelog with multiple changelogs fragments and in multiple passes (run, then run again for new fragments)
• 🟢 collect changelog with hotfix target and a test target
• 🟢 collect changelog with multiline fragment text - it got normalised to 1-line, LGTM