Implements adaptive context strategy selector that chooses the best context
strategy based on plan type, and context fusion that combines results from
multiple strategies with configurable weights.

Features:
- AdaptiveContextSelector: Intelligent strategy selection per plan type
- ContextFusion: Weighted combination of multiple strategy results
- PlanType enumeration: coding, analysis, documentation, refactoring, testing, debugging
- AdaptiveStrategyConfig: YAML-compatible configuration for strategy selection
- FusedResult: Ranked file list with strategy contributions and metadata
- Full type annotations and comprehensive Behave BDD tests

Closes #5255

@@ -0,0 +142,4 @@
+  Scenario: List configured plan types
+    Given I have registered configuration for plan types: "coding", "analysis", "testing"
+    When I list all configured plan types
+    Then I should get plan types: "coding", "analysis", "testing"

@@ -0,0 +77,4 @@
+
+
+@given("I have an adaptive context selector")
+def step_have_selector(context: Any) -> None:

@@ -0,0 +1,336 @@
+"""Adaptive context strategy selector and context fusion implementation.

@@ -0,0 +47,4 @@
+    primary_strategy: str
+    fallback_strategies: list[str] = field(default_factory=list)
+    fusion_weights: dict[str, float] = field(default_factory=dict)
+    use_fusion: bool = False

@@ -0,0 +207,4 @@
+    """Combines results from multiple context strategies with configurable weights."""
+
+    def __init__(self, selector: AdaptiveContextSelector) -> None:
+        """Initialize context fusion.