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

@@ -3,10 +3,13 @@ set -euo pipefail
 
 # mosaic — Unified agent launcher and management CLI
 #
+# AGENTS.md is the single source of truth for all agent sessions.
+# The launcher injects it into every runtime consistently.
+#
 # Usage:
-#   mosaic claude [args...]       Launch Claude Code with SOUL.md injected
-#   mosaic opencode [args...]     Launch OpenCode
-#   mosaic codex [args...]        Launch Codex
+#   mosaic claude [args...]       Launch Claude Code with AGENTS.md injected
+#   mosaic opencode [args...]     Launch OpenCode with AGENTS.md injected
+#   mosaic codex [args...]        Launch Codex with AGENTS.md injected
 #   mosaic init [args...]         Generate SOUL.md interactively
 #   mosaic doctor [args...]       Health audit
 #   mosaic sync [args...]         Sync skills
@@ -22,9 +25,9 @@ mosaic $VERSION — Unified agent launcher
 Usage: mosaic <command> [args...]
 
 Agent Launchers:
-  claude [args...]       Launch Claude Code with SOUL.md injected
-  opencode [args...]     Launch OpenCode
-  codex [args...]        Launch Codex
+  claude [args...]       Launch Claude Code with AGENTS.md injected
+  opencode [args...]     Launch OpenCode with AGENTS.md injected
+  codex [args...]        Launch Codex with AGENTS.md injected
 
 Management:
   init [args...]         Generate SOUL.md (agent identity contract)
@@ -49,6 +52,14 @@ check_mosaic_home() {
   fi
 }
 
+check_agents_md() {
+  if [[ ! -f "$MOSAIC_HOME/AGENTS.md" ]]; then
+    echo "[mosaic] ERROR: ~/.config/mosaic/AGENTS.md not found." >&2
+    echo "[mosaic] Re-run the installer: cd ~/src/mosaic-bootstrap && bash install.sh" >&2
+    exit 1
+  fi
+}
+
 check_soul() {
   if [[ ! -f "$MOSAIC_HOME/SOUL.md" ]]; then
     echo "[mosaic] SOUL.md not found. Running mosaic init..."
@@ -65,31 +76,51 @@ check_runtime() {
   fi
 }
 
+# Ensure AGENTS.md is present at the runtime's native config path.
+# Used for runtimes that don't support CLI prompt injection.
+ensure_runtime_config() {
+  local src="$MOSAIC_HOME/AGENTS.md"
+  local dst="$1"
+  mkdir -p "$(dirname "$dst")"
+  if ! cmp -s "$src" "$dst" 2>/dev/null; then
+    cp "$src" "$dst"
+  fi
+}
+
 # Launcher functions
 launch_claude() {
   check_mosaic_home
+  check_agents_md
   check_soul
   check_runtime "claude"
 
-  # SOUL.md is loaded via ~/.claude/CLAUDE.md directive (pushed by mosaic-link-runtime-assets)
+  # Claude supports --append-system-prompt for direct injection
+  local agents_content
+  agents_content="$(cat "$MOSAIC_HOME/AGENTS.md")"
   echo "[mosaic] Launching Claude Code..."
-  exec claude "$@"
+  exec claude --append-system-prompt "$agents_content" "$@"
 }
 
 launch_opencode() {
   check_mosaic_home
+  check_agents_md
   check_soul
   check_runtime "opencode"
 
+  # OpenCode reads from ~/.config/opencode/AGENTS.md — copy canonical version there
+  ensure_runtime_config "$HOME/.config/opencode/AGENTS.md"
   echo "[mosaic] Launching OpenCode..."
   exec opencode "$@"
 }
 
 launch_codex() {
   check_mosaic_home
+  check_agents_md
   check_soul
   check_runtime "codex"
 
+  # Codex reads from ~/.codex/instructions.md — copy canonical version there
+  ensure_runtime_config "$HOME/.codex/instructions.md"
   echo "[mosaic] Launching Codex..."
   exec codex "$@"
 }