feat: extract config.py, refactor CLI to use event-driven engine, add deprecation shim

- config.py: extracted load_config(), find_workspace_config(),
  load_discussion_config() from cli.py; fixed Windows root detection
- cli.py: run_discussion() now creates DiscussionEngine + EventEmitter,
  subscribes to events with ANSI-color output (identical to before),
  delegates to engine.run()
- backends/custom/run.py: deprecation shim with DeprecationWarning,
  delegates to DiscussionEngine
- 16 new config tests, all 184 tests passing

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

meeting_room/config.py

@@ -0,0 +1,115 @@
+"""Configuration loading for Meeting Room.
+
+Provides load_config(), find_workspace_config(), and load_discussion_config()
+which converts a raw YAML dict into a typed DiscussionConfig.
+"""
+
+from __future__ import annotations
+
+import os
+import re
+
+import yaml
+
+from meeting_room.models import DefaultsConfig, DiscussionConfig, ProviderConfig, RoleConfig
+
+PACKAGE_DIR = os.path.dirname(os.path.abspath(__file__))
+
+
+def load_config(config_path: str | None = None) -> dict:
+    """Load config from file, resolve env vars, apply overrides.
+
+    Parameters
+    ----------
+    config_path : str | None
+        Path to the YAML config file.  When *None*, loads the package
+        default config at ``meeting_room/config/config.yaml``.
+
+    Returns
+    -------
+    dict
+        Raw configuration dictionary (the result of ``yaml.safe_load``).
+    """
+    if config_path is None:
+        config_path = os.path.join(PACKAGE_DIR, "config", "config.yaml")
+
+    with open(config_path, encoding="utf-8") as f:
+        raw = f.read()
+
+    # Resolve ${ENV_VAR} placeholders
+    def _env_replace(match: re.Match) -> str:
+        var = match.group(1)
+        return os.environ.get(var, match.group(0))
+
+    raw = re.sub(r"\$\{(\w+)\}", _env_replace, raw)
+
+    config = yaml.safe_load(raw)
+
+    # Override API keys from environment if set
+    for prov_name, prov_cfg in config.get("providers", {}).items():
+        env_key = f"MEETING_ROOM_{prov_name.upper()}_API_KEY"
+        if os.environ.get(env_key):
+            prov_cfg["api_key"] = os.environ[env_key]
+        env_url = f"MEETING_ROOM_{prov_name.upper()}_BASE_URL"
+        if os.environ.get(env_url):
+            prov_cfg["base_url"] = os.environ[env_url]
+
+    return config
+
+
+def find_workspace_config() -> str | None:
+    """Search for .meeting-room/config.yaml starting from CWD upward.
+
+    Returns
+    -------
+    str | None
+        Absolute path to the workspace config file, or *None* if not
+        found.
+    """
+    current = os.getcwd()
+    while True:
+        candidate = os.path.join(current, ".meeting-room", "config.yaml")
+        if os.path.exists(candidate):
+            return candidate
+        parent = os.path.dirname(current)
+        if parent == current:
+            # Reached filesystem root
+            break
+        current = parent
+    return None
+
+
+def load_discussion_config(config_path: str | None = None) -> DiscussionConfig:
+    """Load config and return a typed DiscussionConfig.
+
+    Convenience wrapper that calls :func:`load_config` and then
+    coerces the raw dict into :class:`DiscussionConfig`.
+
+    Parameters
+    ----------
+    config_path : str | None
+        Path to the YAML config file.  When *None*, loads the package
+        default.
+
+    Returns
+    -------
+    DiscussionConfig
+        Typed configuration object ready for use by DiscussionEngine.
+    """
+    raw = load_config(config_path)
+
+    providers = {
+        name: ProviderConfig(**vals)
+        for name, vals in raw.get("providers", {}).items()
+    }
+    roles = {
+        name: RoleConfig(**vals)
+        for name, vals in raw.get("roles", {}).items()
+    }
+    defaults = DefaultsConfig(**raw.get("defaults", {}))
+
+    return DiscussionConfig(
+        providers=providers,
+        roles=roles,
+        defaults=defaults,
+    )