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/style-20260612-010202.md

@@ -0,0 +1,23 @@
+# style report (check)
+
+- total: 8
+- errors: 8
+- warnings: 0
+- fixed: 0
+- unfixed: 8
+
+## Findings
+
+### agents/agent.py
+- [error] em-dash:30: Em-dash character (U+2014) used in docstring — must be hyphen instead
+- [error] em-dash:1732: Em-dash character (U+2014) used in string literal — must be hyphen instead
+- [error] em-dash:2217: Em-dash character (U+2014) used in instruction string — must be hyphen instead
+- [error] em-dash:2250: Em-dash character (U+2014) used in string literal — must be hyphen instead
+- [error] em-dash:2293: Em-dash character (U+2014) used in string literal — must be hyphen instead
+
+### devplacepy/services/bot/llm.py
+- [error] em-dash:105: Em-dash character (U+2014) used in string literal as replace target — must be hyphen or unicode escape instead
+
+### tests/test_demo.py.bak
+- [error] em-dash:36: Em-dash character (U+2014) used in comments throughout file — must be hyphen instead
+- [error] em-dash:277: Em-dash character (U+2014) used in string literal — must be hyphen instead