AbstractSkill v0.2.0

Added

• Trust-gated activation pipeline: select_skills_for_context, SkillSelection,
  hash-pinned enables, TOCTOU cross-checks, and declared MCP/tool dependency
  surfacing (SkillRequires).
• Fail-closed trust registry: TrustRegistry, evaluate_trust, validation
  records, do-not-use advisories, guidance entries, and lint_registry.
• Curated vendoring catalog: load_catalog, CatalogEntry, SkillCatalog,
  lint_catalog, plus scripts/vendor_skill.py and scripts/refresh_shelf.py.
• Whole-tree hashing and inventory: hash_skill_tree, inspect_skill_dir,
  read_skill_resource with traversal/symlink refusal.
• Derived demand tiering: derive_demand, DemandReport, DemandRow — host
  inventory join for declared tool/MCP requirements (informational; grants
  enforced by the host).
• Tool composition: effective_tools, effective_tools_for_skill (grant ∩
  declared; never widens).
• 14 curated skills under registry/skills/ with byte-pinned validations.
• GitHub Pages documentation workflow (MkDocs Material).

Changed

• Development status bumped to Beta (4).
• Expanded API surface exported from abstractskill.__init__.