Implement a new module-level merge_configs() function in
src/cleveractors/config_utils.py that exposes the §3.1 deep-merge
algorithm as a public, importable API.

Motivation: The internal ReactiveConfigParser._merge_configs() method
mutates its base dict in place and only accepts two dicts. The
CleverThis router (wave 2 of cleveragents/cleveragents-webapp#275)
needs a public merge_configs() to combine platform base configs with
actor configs from the database without side-effects.

Implementation:
- merge_configs(*dicts) accumulates into a fresh dict, applying each
  overlay using the §3.1 rules:
    * Key absent → add via deep copy.
    * Both mappings → deep-merge recursively (_apply_merge helper).
    * Both sequences → extend (existing + deep copy of new).
    * Otherwise → replace with deep copy of new value.
- Zero-argument call returns {}.
- Inputs are never mutated (copy.deepcopy used for all stored values).
- Internal _merge_configs on ReactiveConfigParser is unchanged to avoid
  breaking the existing parse path.

Exports: merge_configs added to cleveractors/__init__.py import and
__all__ so consumers can use 'from cleveractors import merge_configs'.

Tests:
- 11 Behave BDD scenarios in features/merge_configs.feature covering
  zero-args, single-dict copy, absent-key insertion, scalar override,
  recursive mapping merge, sequence append, three-dict chained merge,
  deep nesting, input mutation guard, and type-mismatch replacement.
- 4 Robot Framework integration tests in robot/config.robot verifying
  the same cases at the API boundary.
- 5 ASV benchmarks in benchmarks/merge_configs_benchmark.py for flat,
  nested, sequence, three-way, and zero-arg cases.

Coverage: 97% (config_utils.py: 100%).
Quality gates: ruff lint ✓, pyright strict ✓ (0 errors), all BDD/Robot tests ✓.

ISSUES CLOSED: #11

@@ -0,0 +79,4 @@
+        if key not in base:
+            # Rule: key absent → add it (deep-copy to preserve immutability).
+            base[key] = copy.deepcopy(new_value)
+        else:

@@ -0,0 +83,4 @@
+            existing = base[key]
+            if isinstance(existing, dict) and isinstance(new_value, dict):
+                # Rule: both mappings → deep-merge recursively.
+                _apply_merge(existing, new_value)