docs: add llms.txt for AI/LLM documentation access

[affaan-m]

Summary

Adds a comprehensive llms.txt file to the repository root, providing structured documentation optimized for LLM consumption.

Closes #8816

What's included

The llms.txt covers all major OpenCode features:

• Installation — all package managers, platforms, Docker
• Configuration — opencode.json schema, core settings, env vars
• Agents — built-in agents, custom agent config (JSON + markdown formats), all config options
• Commands — built-in + custom commands, template variables, options
• Tools — all 14 built-in tools, permissions system, custom tool definitions with Zod schema
• Plugins — plugin structure, hook events (25+ events documented), loading methods, examples
• Providers — 75+ providers, config format, model naming, API key setup
• MCP — server configuration, tool permissions
• Ecosystem — plugin categories, discovery resources
• Quick Reference — keyboard shortcuts, commands, file locations, troubleshooting

Why

• Issue [FEATURE]: provide llms.txt and docs as markdown #8816 requests llms.txt + docs as markdown
• Previous PR docs: add llms.txt support for improved AI accessibility #1161 was auto-closed by stale bot — this is a fresh submission
• Makes OpenCode documentation accessible to AI coding assistants and LLM-powered tools
• Follows the llms.txt specification

Test plan

• Verify llms.txt renders correctly on GitHub
• Validate all documented features match current OpenCode behavior
• Check for any outdated configuration examples

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[deadcoder0904]

This uses way too many tokens, no?

https://bun.com/llms.txt seems much smaller & better.

The previous PR did just that with https://delucis.github.io/starlight-llms-txt/

I think this one is equivalent to llms-full.txt

[affaan-m]

This uses way too many tokens, no?

https://bun.com/llms.txt seems much smaller & better.

The previous PR did just that with https://delucis.github.io/starlight-llms-txt/

I think this one is equivalent to llms-full.txt

yeah you're right i meant for it to be llms-full.txt