feat(#696): Config template system — environment-specific overlays

Base config + env overlay merge system:
- config_overlay.py: deep_merge() for dict merging, load_config() for
  loading base + overlay, detect_env() for auto-detection
- config.dev.yaml: local dev (qwen3, verbose, reasoning shown)
- config.prod.yaml: production (claude-opus, compact, privacy)
- config.cron.yaml: headless/cron (no memory, minimal display)
- config.gateway.yaml: messaging gateway (balanced settings)
- tests/test_config_overlay.py: 13 tests, all passing

Usage:
  from config_overlay import load_config
  config = load_config('config.yaml', env='prod')

Overlay naming: config.{env}.yaml
Auto-detect: TIMMY_ENV, HERMES_ENV, ENVIRONMENT, NODE_ENV

tests/test_config_overlay.py

@@ -0,0 +1,136 @@
+"""Tests for config overlay system."""
+import os
+import json
+import tempfile
+import yaml
+import pytest
+
+
+def test_deep_merge_dicts():
+    """Deep merge should recursively merge dicts."""
+    from config_overlay import deep_merge
+    base = {"a": {"b": 1, "c": 2}, "d": 3}
+    overlay = {"a": {"b": 10, "e": 5}}
+    result = deep_merge(base, overlay)
+    assert result == {"a": {"b": 10, "c": 2, "e": 5}, "d": 3}
+
+
+def test_deep_merge_lists_replaced():
+    """Lists should be replaced, not extended."""
+    from config_overlay import deep_merge
+    base = {"items": [1, 2, 3]}
+    overlay = {"items": [4, 5]}
+    result = deep_merge(base, overlay)
+    assert result == {"items": [4, 5]}
+
+
+def test_deep_merge_scalars_overridden():
+    """Scalar values should be overridden by overlay."""
+    from config_overlay import deep_merge
+    base = {"name": "base", "count": 10}
+    overlay = {"name": "override", "count": 20}
+    result = deep_merge(base, overlay)
+    assert result == {"name": "override", "count": 20}
+
+
+def test_deep_merge_none_removes_key():
+    """None in overlay should remove the key from base."""
+    from config_overlay import deep_merge
+    base = {"a": 1, "b": 2, "c": 3}
+    overlay = {"b": None}
+    result = deep_merge(base, overlay)
+    assert result == {"a": 1, "c": 3}
+
+
+def test_deep_merge_empty_overlay():
+    """Empty overlay should return base unchanged."""
+    from config_overlay import deep_merge
+    base = {"a": 1, "b": {"c": 2}}
+    result = deep_merge(base, {})
+    assert result == base
+
+
+def test_load_config_no_env(tmp_path):
+    """Load config without overlay should return base."""
+    from config_overlay import load_config
+    base = {"model": "test", "agent": {"max_turns": 10}}
+    path = tmp_path / "config.yaml"
+    path.write_text(yaml.dump(base))
+    result = load_config(str(path))
+    assert result == base
+
+
+def test_load_config_with_overlay(tmp_path):
+    """Load config with overlay should merge."""
+    from config_overlay import load_config
+    base = {"model": "base", "agent": {"max_turns": 10, "verbose": False}}
+    overlay = {"model": "override", "agent": {"verbose": True}}
+    (tmp_path / "config.yaml").write_text(yaml.dump(base))
+    (tmp_path / "config.dev.yaml").write_text(yaml.dump(overlay))
+    result = load_config(str(tmp_path / "config.yaml"), env="dev")
+    assert result["model"] == "override"
+    assert result["agent"]["max_turns"] == 10
+    assert result["agent"]["verbose"] is True
+
+
+def test_load_config_missing_overlay(tmp_path):
+    """Missing overlay should silently return base."""
+    from config_overlay import load_config
+    base = {"model": "base"}
+    (tmp_path / "config.yaml").write_text(yaml.dump(base))
+    result = load_config(str(tmp_path / "config.yaml"), env="nonexistent")
+    assert result == base
+
+
+def test_find_config(tmp_path):
+    """find_config should locate base and overlay."""
+    from config_overlay import find_config
+    base = tmp_path / "config.yaml"
+    base.write_text("a: 1")
+    overlay = tmp_path / "config.prod.yaml"
+    overlay.write_text("a: 2")
+    b, o = find_config(str(base), "prod")
+    assert b == base
+    assert o == overlay
+
+
+def test_list_overlays(tmp_path):
+    """list_overlays should find all overlay files."""
+    from config_overlay import list_overlays
+    (tmp_path / "config.yaml").write_text("a: 1")
+    (tmp_path / "config.dev.yaml").write_text("a: 2")
+    (tmp_path / "config.prod.yaml").write_text("a: 3")
+    overlays = list_overlays(str(tmp_path / "config.yaml"))
+    envs = [o['env'] for o in overlays]
+    assert 'dev' in envs
+    assert 'prod' in envs
+
+
+def test_detect_env_from_var(tmp_path, monkeypatch):
+    """detect_env should check TIMMY_ENV first."""
+    from config_overlay import detect_env
+    monkeypatch.setenv("TIMMY_ENV", "prod")
+    assert detect_env() == "prod"
+
+
+def test_detect_env_fallback(tmp_path, monkeypatch):
+    """detect_env should fall back through vars."""
+    from config_overlay import detect_env
+    monkeypatch.delenv("TIMMY_ENV", raising=False)
+    monkeypatch.delenv("HERMES_ENV", raising=False)
+    monkeypatch.setenv("ENVIRONMENT", "cron")
+    assert detect_env() == "cron"
+
+
+def test_real_config_overlay():
+    """Test against actual config files in the repo."""
+    import sys
+    sys.path.insert(0, os.path.dirname(os.path.dirname(__file__)))
+    from config_overlay import load_config
+
+    base_path = os.path.join(os.path.dirname(__file__), '..', 'config.yaml')
+    if os.path.exists(base_path):
+        config = load_config(base_path, env='dev')
+        assert config['model']['default'] == 'qwen3:30b'  # dev overrides
+        assert config['agent']['max_turns'] == 50  # dev overrides
+        assert 'terminal' in config  # base keys preserved