refactor: AGENTS.md is the single source of truth for all runtimes

- Create ~/.config/mosaic/AGENTS.md as the canonical universal agent config
- Runtime adapters (CLAUDE.md, opencode/AGENTS.md, codex/instructions.md) are
  now thin pointers that say "READ ~/.config/mosaic/AGENTS.md"
- mosaic claude: injects AGENTS.md via --append-system-prompt
- mosaic opencode/codex: copies AGENTS.md to runtime config path before launch
- mosaic-link-runtime-assets: pushes thin pointers for direct launch fallback

AGENTS.md is runtime-agnostic. All runtimes get the same standards.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bin/mosaic-link-runtime-assets.ps1

@@ -77,7 +77,7 @@ foreach ($p in $legacyPaths) {
     Remove-LegacyPath $p
 }
 
-# Sync runtime config files (copy, not link)
+# Claude-specific runtime files (settings, hooks — CLAUDE.md is now a thin pointer)
 $runtimeFiles = @("CLAUDE.md", "settings.json", "hooks-config.json", "context7-integration.md")
 foreach ($rf in $runtimeFiles) {
     $src = Join-Path $MosaicHome "runtime\claude\$rf"