agent: inject AGENTS.md from cwd into user context

If AGENTS.md exists in the working directory, its contents are injected
as a user message at session start and after compaction (via spawnContext).
Not part of the system prompt — lives in conversation history.

agent/turn.go

@@ -420,6 +420,12 @@ func (ag *Agent) spawnContext(ctx context.Context) string {
 	if result.Context != "" {
 		parts = append(parts, result.Context)
 	}
+	// Inject AGENTS.md from the working directory if it exists.
+	if cwd := ag.effectiveCwd(); cwd != "" {
+		if data, err := os.ReadFile(cwd + "/AGENTS.md"); err == nil && len(data) > 0 {
+			parts = append(parts, string(data))
+		}
+	}
 	return strings.Join(parts, "\n\n---\n\n")
 }