skillscript-runtime v0.21.1

The MCP server now teaches a connecting agent how to use it — no CLAUDE.md
copy-paste, and it works for agents with no shared memory substrate.

• initialize carries an instructions field — the canonical agent-usage
  block (SKILLSCRIPT_USAGE_INSTRUCTIONS, a static backend-agnostic string in
  help-content.ts). Every connecting agent learns the workflow at session
  start: skill_list first → skill_preflight before composing → author when
  a routine repeats (lands Draft, a human approves) → runtime_capabilities.
• Capabilities reframed as the author's menu — the block presents the wired
  connectors / models / shell-allowlist as what's capturable into a skill, not
  just a defensive "don't assume a backend" check; if a needed capability isn't
  wired, the agent asks the operator to add it.
• docs/adopter-agent-guide §1 reframed — the runtime delivers the usage
  contract automatically, so an adopter's agent file can be minimal; the
  paste-block is now a fallback that mirrors the server's authoritative copy.