Agent quickstart: agent.md, llms.txt, --json output and firstrun doctor

Any coding agent can now read one URL and run Doc-test, Doc-audit and receipts without a human in the loop.

• docs/agent.md is the canonical agent quickstart: preflight, install, recipes, JSON keys, exit and error codes, safety rules and limits.
• docs/llms.txt, docs/llms-full.txt (built by scripts/build_llms_full.py) and docs/robots.txt. netlify.toml serves the agent files as UTF-8 text with Access-Control-Allow-Origin and a 5 minute cache, and opens CORS on audits/, runs/ and receipts/.
• firstrun run, audit, receipt and prereqs take --json: one JSON object on stdout with ok, command, exit_code and error {code, message, fix}. Human lines go to stderr.
• firstrun doctor [--for all|test|audit] [--json] checks Python, git, Docker, each key (never its value) and the network, with a fix for each failed check.
• AGENTS.md gains build, test and safety rules. skills/firstrun/SKILL.md is an Agent Skill. README and evidence point at all of them.
• The homepage footer, Doc-test and Doc-audit link agent.md. The two tool pages copy an agent prompt that starts with agent.md.