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>

runtime/codex/instructions.md

@@ -2,18 +2,10 @@
 
 ## MANDATORY — Read Before Any Response
 
-BEFORE responding to any user message, READ these files in order:
+BEFORE responding to any user message, READ `~/.config/mosaic/AGENTS.md`.
 
-1. `~/.config/mosaic/SOUL.md` (identity and behavioral contract)
-2. `~/.config/mosaic/STANDARDS.md` (machine-wide standards)
-3. Project-local `AGENTS.md` (project operations and workflows)
+That file is the universal agent configuration. It contains the full load order,
+development standards, skills, and workflows. Do NOT respond until you have loaded it.
 
-Do NOT respond to the user until you have loaded all three.
-
-This file is a Codex adapter layer. It does not replace project guidance.
-
-## Runtime Behavior
-
-- Favor repo lifecycle scripts under `scripts/agent/` for start/end rituals.
-- Keep instructions and quality gates aligned with Mosaic standards.
-- Use `~/.config/mosaic/skills/` for domain expertise when available.
+This file (`~/.codex/instructions.md`) exists only as a fallback for direct `codex` launches.
+For full injection, use `mosaic codex` instead.