v0.2.0

chore(release): bump to 0.2.0 + release notes

v0.1.0 (tag at d0383cd) is taken and HEAD is +19 with this session's prefpane UX + native EdDSA signer
+ appcast pipeline, so the next release needs a new version. Bump MT2_VERSION 0.1.0 -> 0.2.0 (the .pkg
version, the CI tag==version check, and the appcast all derive from it) and add
docs/release-notes/v0.2.0.md (required by the notes gate; becomes the appcast <description>).

Notes describe only user-reachable changes: the updater is bundled and the release feed is signed, but
the in-pane "Check for Updates" control (Phase 4) is not wired yet (mt2_launch_updater is still
uncalled), so the self-update line says "groundwork" rather than claiming a working button.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>