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

@@ -69,7 +69,7 @@ for p in "${legacy_paths[@]}"; do
   remove_legacy_path "$p"
 done
 
-# Runtime files are synced as regular files (not symlinks) to reduce path confusion.
+# Claude-specific runtime files (settings, hooks — NOT CLAUDE.md which is now a thin pointer)
 for runtime_file in \
   CLAUDE.md \
   settings.json \
@@ -80,13 +80,13 @@ for runtime_file in \
   copy_file_managed "$src" "$HOME/.claude/$runtime_file"
 done
 
-# OpenCode runtime adapter (optional, copied when present).
+# OpenCode runtime adapter (thin pointer to AGENTS.md)
 opencode_adapter="$MOSAIC_HOME/runtime/opencode/AGENTS.md"
 if [[ -f "$opencode_adapter" ]]; then
   copy_file_managed "$opencode_adapter" "$HOME/.config/opencode/AGENTS.md"
 fi
 
-# Codex runtime adapter (optional, copied when present).
+# Codex runtime adapter (thin pointer to AGENTS.md)
 codex_adapter="$MOSAIC_HOME/runtime/codex/instructions.md"
 if [[ -f "$codex_adapter" ]]; then
   mkdir -p "$HOME/.codex"