docs: add OpenCode engine guide and update references

Add comprehensive external-facing documentation for the OpenCode engine:
- New guide: docs/src/content/docs/guides/opencode.md (460 lines)
  - Quick start, auth/providers, network security, MCP support
  - Example workflows, engine comparison, known limitations
- Update engines.md: add OpenCode to available agents list
- Update auth.mdx: add OpenCode authentication reference
- Update network.md: add OpenCode domain config + dynamic provider docs
- Update astro.config.mjs: add sidebar link

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Mossaka

docs/astro.config.mjs

@@ -182,6 +182,7 @@ export default defineConfig({
 						{ label: 'GitHub Actions Primer', link: '/guides/github-actions-primer/' },
 						{ label: 'Reusing Workflows', link: '/guides/packaging-imports/' },
 						{ label: 'Self-Hosted Runners', link: '/guides/self-hosted-runners/' },
+						{ label: 'OpenCode Engine', link: '/guides/opencode/' },
 						{ label: 'Using MCPs', link: '/guides/mcps/' },
 						{ label: 'Web Search', link: '/guides/web-search/' },
 					],