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

config.dev.yaml

@@ -0,0 +1,14 @@
+# Dev environment overlay
+# Merges with config.yaml — these keys override the base
+model:
+  default: qwen3:30b
+  provider: custom
+agent:
+  max_turns: 50
+  verbose: true
+display:
+  show_reasoning: true
+  streaming: true
+  show_cost: true
+terminal:
+  timeout: 300