chore: add changelog for release notes

[ovitrif]

This PR adds a CHANGELOG.md system following Keep a Changelog format so developers can document user-facing changes as part of their PR workflow. Mirrors the setup from synonymdev/bitkit-android#874.

Description

1. Creates CHANGELOG.md with an empty [Unreleased] section (no backfilling)
2. Adds an HTML comment to the PR template reminding contributors to update the changelog
3. Adds changelog rules to AGENTS.md
4. Updates /release command to finalize changelog entries into versioned sections and use them as primary source for store release notes
5. Updates /pr command to backfill PR numbers into changelog entries after PR creation

Linked Issues/Tasks

• chore: add changelog for release notes bitkit-android#874

Screenshot / Video

N/A

QA Notes

1. Verify CHANGELOG.md follows keepachangelog.com format
   • ## [Unreleased] section present
   • Compare link at bottom points to v2.1.2...HEAD
2. Verify PR template comment is invisible in rendered view but visible in edit mode
3. Verify changelog rules in AGENTS.md match the Android counterpart
4. Verify /release command changes:
   • New Step 2c reads and finalizes changelog
   • Step 3 includes CHANGELOG.md in the version bump commit
   • Step 6 uses changelog as primary source for store notes
5. Verify /pr command Step 8b backfills PR numbers into changelog entries

[claude]

Code review

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

[piotr-iohk]

LGTM