docs: v0.21.7 release notes + refresh stale CHANGELOG.md

The CHANGELOG.md was stuck on a single '[Unreleased] - v3.3.1' entry
using an internal version codename (the public release was v0.17.0,
shipped mid-May) with implementation jargon users could not contextualize.
Replace it with a thin pointer to the GitHub Releases page (which is the
canonical user-facing changelog) plus a per-release-line highlights map
that summarizes themes without duplicating the full release notes.

Add v0.21.7 release notes covering:
  - Compressor cross-process safety (GH #91 fix)
  - Unified disable: true agent off-switch
  - Startup release announcement dialog
  - Auto-search filter for ignored plugin-internal messages
  - TUI execute-threshold display fixes (GH #90)
  - Migration warning silenced for no-op enabled=true legacy keys

Both files target a release; no runtime change.