feat: warn at startup on unrecognized mcbot.conf keys/sections

[sg]

Summary

mcbot.conf isn't strictly validated, so a typo'd or stale setting was silently ignored. The bot now logs each unrecognized section/key loudly at WARNING on startup:

[WARNING] mcbot.conf: [bot] unrecognized key 'foo' (ignored)
[WARNING] mcbot.conf: unrecognized section [bogus] (ignored)
[WARNING] mcbot.conf: [bot] 'rx_log_decrypt' is deprecated and ignored (safe to remove)

It's non-fatal — the bot still starts (a typo shouldn't brick it).

Changes

• mcbot.py:
  • _KNOWN_CONFIG_KEYS — recognized keys per section, kept in sync with what load_config reads.
  • _DYNAMIC_CONFIG_SECTIONS (env, channels) — arbitrary keys allowed, not checked (their keys are user data, not option names).
  • _DEPRECATED_CONFIG_KEYS (bot.rx_log_decrypt) — reported as deprecated/“safe to remove” rather than a generic unknown key, so upgraders get a friendly nudge.
  • _check_unknown_config_keys(parser) collects warnings into cfg.config_warnings; run() logs them after the startup banner.
• tests/test_config_keys.py — valid config yields zero warnings (no false positives); unknown key, unknown section, and deprecated rx_log_decrypt are each flagged; [env]/[channels] keys are not.
• Bot_Usage.md — documents the behavior.

Maintenance note

_KNOWN_CONFIG_KEYS must be updated when a new setting is added; it sits right above load_config with a reminder comment.

Testing

py_compile clean; new test passes; all five existing suites pass.