ErrorEnvelope component named; 404s return markdown

Added

• OpenAPI spec now registers the error envelope as a named ErrorEnvelope component — previously inlined anonymously at every error response, so code generators and API tooling can reference the { error: { code, type, message, details? } } shape directly.
• Nonexistent API and docs paths return a text/markdown 404 body — with pointers to llms.txt, the docs index, the REST API base, and the OpenAPI spec — so AI agents and tools hitting a wrong endpoint get actionable next steps instead of an HTML shell or empty body.
• llms.txt gains a "When to use" section: what questions releases.sh answers well (what shipped across a stack, who shipped integrations, roadmap/product research), the fastest call paths (CLI, MCP tools, REST), and an explicit non-goal.