fix(compression): keep default protect_first_n at 3 + align ABC

Follow-up on the salvaged feat commit:

- Keep the constructor / config / yaml-example default at 3 so existing
  gateway and CLI users see no behavioural change. PR #13754 (which this
  builds on) had lowered the default to 2 to chase pre-feature parity in
  the system-prompt-present case, at the cost of quietly halving the
  protected head for the gateway path (which strips the system prompt
  before calling compress()). With the new "system prompt is implicit"
  semantics, default 3 gives every caller a stable head shape.
- agent/context_engine.py: bring the ABC's protect_first_n docstring in
  line with the new semantics so plugin context engines interpret the
  config key the same way the built-in compressor does.
- tests: adjust the default-value test (3, not 2) and a stale comment;
  per-test protect_first_n=2/3/1 values added in PR #13754 stay as-is
  since those tests fix concrete head shapes.

tests/agent/test_context_compressor.py

@@ -1299,14 +1299,16 @@ class TestSummaryTargetRatio:
             c = ContextCompressor(model="test", quiet_mode=True)
         assert c.protect_last_n == 20
 
-    def test_default_protect_first_n_is_2(self):
-        """Default protect_first_n is 2 (system + 2 extra non-system messages =
-        3 protected messages total, preserving the pre-feature behaviour where
-        protect_first_n was hardcoded to protect 3 head messages total).
+    def test_default_protect_first_n_is_3(self):
+        """Default protect_first_n is 3 (system + 3 extra non-system messages =
+        4 protected messages total when a system prompt is present). With the
+        new semantics, the constructor default is 3 — the system prompt is
+        always implicitly protected ON TOP OF protect_first_n non-system
+        messages.
         """
         with patch("agent.context_compressor.get_model_context_length", return_value=100_000):
             c = ContextCompressor(model="test", quiet_mode=True)
-        assert c.protect_first_n == 2
+        assert c.protect_first_n == 3
 
     def test_protect_first_n_override(self):
         """protect_first_n=0 should be honoured — for users who rely on rolling
@@ -1342,8 +1344,8 @@ class TestSummaryTargetRatio:
         assert result[0]["content"].startswith("System prompt")
         # The first user/assistant exchange (msg 0, msg 1) should NOT be pinned
         # as head verbatim — those would have been summarized or absorbed.
-        # Under default protect_first_n=2, result[1] and result[2] would be
-        # the literal "msg 0" / "msg 1"; with protect_first_n=0 they aren't.
+        # Under default protect_first_n=3, result[1..3] would be the literal
+        # "msg 0" / "msg 1" / "msg 2"; with protect_first_n=0 they aren't.
         assert result[1].get("content") != "msg 0"
         # Last 2 messages are tail-protected under protect_last_n=2
         assert result[-1]["content"] == msgs[-1]["content"]