feat: adapt review budget to diff size

pilot/review/budget.py

@@ -15,6 +15,13 @@ DEFAULTS = {
     "max_duration_seconds": 480,
 }
 
+PROFILES = (
+    # (changed lines threshold, steps, total tokens, output tokens, seconds)
+    (2_000, 80, 1_200_000, 80_000, 1_800),
+    (800, 60, 800_000, 60_000, 1_200),
+    (200, 40, 400_000, 40_000, 900),
+)
+
 
 @dataclass(frozen=True)
 class Budget:
@@ -63,6 +70,34 @@ class Budget:
             price_target=price_target,
         )
 
+    @classmethod
+    def for_review(cls, config: dict | None, diff: str) -> "Budget":
+        """Choose safe headroom from diff size, unless config is explicit."""
+        if isinstance((config or {}).get("budget"), dict):
+            return cls.from_config(config)
+        changed_lines = _changed_line_count(diff)
+        for threshold, steps, tokens, output, seconds in PROFILES:
+            if changed_lines >= threshold:
+                return cls.from_config({
+                    **(config or {}),
+                    "budget": {
+                        "max_steps": steps,
+                        "max_total_tokens": tokens,
+                        "max_output_tokens": output,
+                        "max_duration_seconds": seconds,
+                    },
+                })
+        return cls.from_config(config)
+
+
+def _changed_line_count(diff: str) -> int:
+    """Count changed lines without treating hunk headers as additions."""
+    return sum(
+        1 for line in diff.splitlines()
+        if (line.startswith("+") and not line.startswith("+++"))
+        or (line.startswith("-") and not line.startswith("---"))
+    )
+
 
 def _env_int(name: str, default: int) -> int:
     try: