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.ps1

@@ -1,9 +1,12 @@
 # mosaic.ps1 — Unified agent launcher and management CLI (Windows)
 #
+# 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
@@ -19,9 +22,9 @@ 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)
@@ -43,6 +46,15 @@ function Assert-MosaicHome {
     }
 }
 
+function Assert-AgentsMd {
+    $agentsPath = Join-Path $MosaicHome "AGENTS.md"
+    if (-not (Test-Path $agentsPath)) {
+        Write-Host "[mosaic] ERROR: ~/.config/mosaic/AGENTS.md not found." -ForegroundColor Red
+        Write-Host "[mosaic] Re-run the installer."
+        exit 1
+    }
+}
+
 function Assert-Soul {
     $soulPath = Join-Path $MosaicHome "SOUL.md"
     if (-not (Test-Path $soulPath)) {
@@ -60,6 +72,18 @@ function Assert-Runtime {
     }
 }
 
+function Ensure-RuntimeConfig {
+    param([string]$Dst)
+    $src = Join-Path $MosaicHome "AGENTS.md"
+    $parent = Split-Path $Dst -Parent
+    if (-not (Test-Path $parent)) { New-Item -ItemType Directory -Path $parent -Force | Out-Null }
+    $srcHash = (Get-FileHash $src -Algorithm SHA256).Hash
+    $dstHash = if (Test-Path $Dst) { (Get-FileHash $Dst -Algorithm SHA256).Hash } else { "" }
+    if ($srcHash -ne $dstHash) {
+        Copy-Item $src $Dst -Force
+    }
+}
+
 if ($args.Count -eq 0) {
     Show-Usage
     exit 0
@@ -71,23 +95,31 @@ $remaining = if ($args.Count -gt 1) { $args[1..($args.Count - 1)] } else { @() }
 switch ($command) {
     "claude" {
         Assert-MosaicHome
+        Assert-AgentsMd
         Assert-Soul
         Assert-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
+        $agentsContent = Get-Content (Join-Path $MosaicHome "AGENTS.md") -Raw
         Write-Host "[mosaic] Launching Claude Code..."
-        & claude @remaining
+        & claude --append-system-prompt $agentsContent @remaining
     }
     "opencode" {
         Assert-MosaicHome
+        Assert-AgentsMd
         Assert-Soul
         Assert-Runtime "opencode"
+        # OpenCode reads from ~/.config/opencode/AGENTS.md
+        Ensure-RuntimeConfig (Join-Path $env:USERPROFILE ".config\opencode\AGENTS.md")
         Write-Host "[mosaic] Launching OpenCode..."
         & opencode @remaining
     }
     "codex" {
         Assert-MosaicHome
+        Assert-AgentsMd
         Assert-Soul
         Assert-Runtime "codex"
+        # Codex reads from ~/.codex/instructions.md
+        Ensure-RuntimeConfig (Join-Path $env:USERPROFILE ".codex\instructions.md")
         Write-Host "[mosaic] Launching Codex..."
         & codex @remaining
     }