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-20260612-010804.md

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+# style report (fix)
+
+- total: 7
+- errors: 5
+- warnings: 0
+- fixed: 5
+- unfixed: 2
+
+## Findings
+
+### agents/agent.py
+- [fixed] no-em-dash:30: Em-dash character (U+2014) used instead of hyphen in docstring
+  fix: Replaced em-dash with hyphen in docstring about Chrome-accurate header order
+- [fixed] no-em-dash:1732: Em-dash character (U+2014) used instead of hyphen in advice string literal
+  fix: Replaced em-dash with hyphen in plan confidence advice string
+- [fixed] no-em-dash:2217: Em-dash character (U+2014) used instead of hyphen in SYSTEM_PROMPT string
+  fix: Replaced em-dash with hyphen in operating protocol text
+- [fixed] no-em-dash:2250: Em-dash character (U+2014) used instead of hyphen in _with_datetime() f-string
+  fix: Replaced em-dash with hyphen in datetime message string
+- [fixed] no-em-dash:2293: Em-dash character (U+2014) used instead of hyphen in argparse description
+  fix: Replaced em-dash with hyphen in argument parser description
+
+### devplacepy/services/bot/llm.py
+- [info] no-em-dash:105: Em-dash characters in llm.py replace() pattern are intentional data processing logic; not changed
+
+### devplacepy/static/uploads/attachments/1a/64/1a640917-f599-4fce-8dc4-2babdb25f716.py
+- [info] no-em-dash:3: Em-dash in user-uploaded attachment file; not project source