Add CHANGELOG.md with an Unreleased section and a verified 0.2.35 baseline
from published release notes. Link earlier history rather than reconstructing
it from commit titles. Include the changelog in the widget archive.

Validate changelog structure and its latest version against metadata in
make check. Before publishing a tag, require matching metadata and changelog
versions and no unassigned Unreleased entries. Extract that release's body
for GitHub notes instead of maintaining a separate generated summary.

Documentation impact:
Add the changelog to README and the documentation index. Document the release
procedure, require same-change agent maintenance, and add PR changelog impact.
Unreleased records the new packaged history and recent user-guide changes.
Internal account-request refactoring adds no user-facing release claim.

Verification:
- make check passed: 823 QML tests, 3 desktop-style QML tests, 46 Python tests,
  and repository lint, static, and AppStream metadata checks.
- make package passed; packaged changelog content and README links verified.
- actionlint .github/workflows/ci.yml passed.
- Seven changelog tests cover extraction, malformed history, metadata/tag
  mismatch, unassigned work, CLI failure output, and workflow wiring.
- Simulated preparation of a future release in temporary files and verified
  that extraction excludes the older release.
- git diff --check passed.

No version bump, tag publication, or existing GitHub release edits. Existing
feature issues remain open; no unresolved gaps in this change.