Add coding-agent guidance and llms.txt decision rules

[autocarl]

Summary

• Adds a new For Coding Agents guide with decision rules, minimal implementation snippets, MCP guidance, structured-output guidance, and copy/paste AGENTS.md / CLAUDE.md / Cursor / Cline instructions.
• Updates llms.txt so agent-readable entry points include the new guide, explicit decision rules, common agent tasks, and the Repl.Mcp wording clarification.
• Clarifies across MCP docs that Repl.Mcp is the component used to build MCP servers; the app built with Repl is the server an agent host launches.

Refs yllibed/repl#36.

Verification

• git diff --check origin/main...HEAD — passed.
• ASTRO_TELEMETRY_DISABLED=1 XDG_CONFIG_HOME=/workspace/.config npm run astro -- build — passed, including /getting-started/for-coding-agents/, with a local generated API-reference stub for ignored API docs.
• Full npm run build was attempted locally, but DocFX metadata generation failed against the local checked-out Repl source before Astro build.
• After opening the draft PR, gh pr checks reported no checks on the branch. Current workflow triggers are push to main, workflow_dispatch, repository_dispatch, and scheduled deploy; there is no pull_request CI trigger in this repo yet.