feat: add autonomous maintenance agent fleet with shared engine and validator

Add the `agents/` directory containing a fleet of autonomous AI maintenance agents for codebase consistency, including a shared async engine (`agent.py`), a dependency-free validator (`validator.py`), and specialized agents for security, audit, docs, style, frontend, SEO, test coverage, and more. Wire the fleet into the Makefile with `--fix`/`--check` modes, document the architecture in `AGENTS.md`, `CLAUDE.md`, and `README.md`, and enforce a hard rule that the `agents/` directory itself is off-limits to any code modification to preserve detection data.

agents/reports/seo-20260612-013402.json

@@ -0,0 +1,18 @@
+{
+  "agent": "seo",
+  "mode": "check",
+  "started_at": "2026-06-12T01:34:02.164037",
+  "finished_at": "2026-06-12T01:34:16.966318",
+  "summary": {
+    "total": 0,
+    "errors": 0,
+    "warnings": 0,
+    "fixed": 0,
+    "unfixed": 0
+  },
+  "cost": {
+    "run_usd": "0.00151031",
+    "session_usd": "0.00151031"
+  },
+  "findings": []
+}