Implements SlidingWindowStrategy class that satisfies the ContextStrategy
protocol for the ACMS pipeline. The strategy limits token usage by keeping
only the most recent N messages or tokens in context, which is critical for
long-running agent sessions that would otherwise exceed LLM context limits.

Key features:
- Configurable window_size (int) and window_mode ('messages' | 'tokens')
- Messages mode: keeps the most recent window_size non-system fragments
- Tokens mode: keeps the most recent fragments within the token budget
- System prompt preservation: fragments with role='system' are always kept
- Registered in the plugin registry under key 'sliding_window'
- Input validation: window_size must be positive, window_mode must be valid
- Full BDD test coverage with 22 scenarios across all acceptance criteria

ISSUES CLOSED: #9995

@@ -0,0 +60,4 @@
+      | project://app/b.py    | message 2 | 0.6   | 10     | 3     | user    |
+    And a sliding window budget with max_tokens 1000 and reserved_tokens 0
+    When I apply the SlidingWindowStrategy
+    Then 2 fragments should be returned by sliding window strategy

@@ -0,0 +72,4 @@
+def step_sliding_window_strategy(context: Context, size: int, mode: str) -> None:
+    context.sw_strategy = SlidingWindowStrategy(
+        window_size=size,
+        window_mode=mode,  # type: ignore[arg-type]