Config schema, example config and validating loader

config/schema.json mirrors design §5 (terminals, executors, projects with routing,
retry, timeouts). agentd/config.py loads YAML, validates with jsonschema when
available or a small built-in draft-07 subset, adds cross-reference checks, and
maps token_key to the environment variable injected by agent-access exec.

agentd/__init__.py

@@ -0,0 +1,4 @@
+"""plane-agentd: Plane-driven dispatcher for CLI agents (see docs/design.md)."""
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parent.parent