fix: sanitizeHistoryForProvider now repairs orphaned tool_use blocks

Calls repairOrphanedToolPairs as the first step so that assistant messages
with tool_calls that lack matching tool_results get synthetic results
injected. This prevents Anthropic API 400 errors.

Fixes #475

pkg/agent/context.go

@@ -221,6 +221,9 @@ func sanitizeHistoryForProvider(history []providers.Message) []providers.Message
 		return history
 	}
 
+	// Repair orphaned tool_use/tool_result pairs before further sanitization.
+	history = repairOrphanedToolPairs(history)
+
 	sanitized := make([]providers.Message, 0, len(history))
 	for _, msg := range history {
 		switch msg.Role {

pkg/agent/sanitize_test.go

@@ -78,6 +78,25 @@ func TestRepairOrphanedToolPairs_OrphanToolResultDropped(t *testing.T) {
 	}
 }
 
+func TestSanitizeHistoryForProvider_OrphanToolUseRepaired(t *testing.T) {
+	history := []providers.Message{
+		{Role: "user", Content: "do something"},
+		{
+			Role: "assistant", Content: "calling tool",
+			ToolCalls: []providers.ToolCall{{ID: "call_99", Name: "exec"}},
+		},
+	}
+	sanitized := sanitizeHistoryForProvider(history)
+
+	if len(sanitized) != 3 {
+		t.Fatalf("expected 3 messages, got %d", len(sanitized))
+	}
+	if sanitized[2].Role != "tool" || sanitized[2].ToolCallID != "call_99" {
+		t.Errorf("expected synthetic tool_result for call_99, got role=%q id=%q",
+			sanitized[2].Role, sanitized[2].ToolCallID)
+	}
+}
+
 func TestRepairOrphanedToolPairs_EmptyInput(t *testing.T) {
 	repaired := repairOrphanedToolPairs(nil)
 	if len(repaired) != 0 {