AbstractSkill v0.2.1

Fixed

• Boolean flags in registry and catalog data (ValidationRecord.delivered_via_map,
  CatalogEntry.vendored) are now parsed strictly: "false", "no", "0" and
  "off" are false instead of being read as true, and unrecognised values raise
  SkillValidationError instead of being silently coerced.
• FilesystemSkillLoader skips a skill root it cannot list (with a #FALLBACK
  warning) instead of aborting discovery, and an unreadable SKILL.md raises
  SkillParseError instead of a raw OSError.

Changed

• Documentation site moved to https://www.lpalbou.info/AbstractSkill/; the
  project Documentation URL points there.
• CI now builds the documentation site on every push and pull request, and the
  release workflow publishes the docs after a successful release.