Todo Companion 0.6.0

First public GitHub Release of the current product: hierarchical task semantics,
stronger priority coloring, search hardening, a marketing landing site, and an
automated release pipeline that attaches a verified .vsix to each v* tag.

Added

• Hierarchical completion prerequisites: a parent cannot be marked done while
  open or in-progress checkbox children remain (detail lines are not blockers).
• Product landing site (landing/): Living File hero, product story, theme
  toggle, isometric workflow scene, and install CTAs pointed at GitHub Releases.
• Release automation: tag vX.Y.Z runs typecheck, unit tests, VSIX package
  verification, and creates a GitHub Release with the .vsix asset
  (see docs/RELEASE.md).
• GitHub Pages deploy for the landing site on pushes to main under landing/.

Changed

• Priority presentation is clearer: stronger hierarchy for @critical / @high /
  @low, and completed low-priority lines fade consistently with the rest of the
  done styling.
• Internal structure: shared hierarchy model, leaner activation, shared document
  replace helpers, and modular landing story scripts/styles (behavior unchanged
  for users).

Fixed

• Semantic search: epoch-guarded config invalidation and batch delete handling so
  index updates cannot race into stale hits.
• Completion target boundaries respected when cycling or completing items.
• Hierarchy command coverage exercised against an active editor in tests.
• VSIX packaging excludes the landing site and other repo-only trees so the
  download stays ~4 MB (runtime + ONNX WASM only).