fix(pipeline): include tool-schema tokens in overhead + dynamic compact max_tokens

- Add TokenCounter.CountToolSchemas() to measure JSON schema size for all tools
- Include tool schemas in OverheadTokens calculation for accurate context usage
- Implement dynamic max_tokens: in/25 clamp [1024, 8192] for compaction
- Add characterization tests: count_tool_schemas_test.go
- Add overhead verification tests: context_stage_overhead_test.go, context_stage_tool_overhead_test.go
- Add integration tests: context_stage_integration_test.go
- Add compact tests: loop_compact_dynamic_max_test.go, loop_compact_max_tokens_test.go
- Add sanitize tests: loop_history_sanitize_max_tokens_test.go
- Add integration test: loop_compact_integration_test.go

internal/agent/loop_compact_dynamic_max_test.go

@@ -0,0 +1,26 @@
+package agent
+
+import "testing"
+
+// TestDynamicSummaryMax validates boundary cases for dynamicSummaryMax.
+// Formula: out = in/25, clamped to [1024, 8192].
+func TestDynamicSummaryMax(t *testing.T) {
+	cases := []struct {
+		input int
+		want  int
+	}{
+		{0, 1024},       // zero → floor
+		{20000, 1024},   // 20000/25=800 → below floor, clamped
+		{25000, 1024},   // 25000/25=1000 → below floor, clamped
+		{26000, 1040},   // 26000/25=1040 → just above floor
+		{100000, 4000},  // 100000/25=4000 → mid-range
+		{204800, 8192},  // 204800/25=8192 → exactly at cap
+		{500000, 8192},  // 500000/25=20000 → above cap, clamped
+	}
+	for _, tc := range cases {
+		got := dynamicSummaryMax(tc.input)
+		if got != tc.want {
+			t.Errorf("dynamicSummaryMax(%d) = %d, want %d", tc.input, got, tc.want)
+		}
+	}
+}