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.

cli-config.yaml.example

@@ -372,9 +372,9 @@ compression:
   # opening turns may not match how you want the session framed over time.
   # Set to 0 to preserve ONLY the system prompt (plus the rolling summary
   # and recent tail) — the cleanest configuration for long-running sessions.
-  # Default 2 preserves the system prompt plus the first user/assistant
-  # exchange (≈ 3 messages total when a system prompt is present).
-  protect_first_n: 2
+  # Default 3 preserves the system prompt plus the first three non-system
+  # head messages, matching the pre-feature behaviour.
+  protect_first_n: 3
 
   # To pin a specific model/provider for compression summaries, use the
   # auxiliary section below (auxiliary.compression.provider / model).