feat(server): add DCP clean-room context pruning

- Deduplication: removes consecutive identical tool_call+tool_result pairs
- Purge-errors: removes failed/empty tool results
- Transform orchestrator runs strategies in sequence pre-payload
- Wired into turn.ts before buildMessagesPayload
- Clean-room reimplementation (AGPL reference: behavior only)

apps/server/src/services/inference/dcp/strategies/deduplication.ts

@@ -0,0 +1,50 @@
+import type { DcpMessage } from '../messages.js';
+
+export function deduplicate(messages: DcpMessage[]): { messages: DcpMessage[]; stats: { removedCount: number; freedTokens: number } } {
+  const result: DcpMessage[] = [];
+  let removedCount = 0;
+  let freedTokens = 0;
+  let i = 0;
+
+  while (i < messages.length) {
+    const current: DcpMessage = messages[i]!;
+    const next = messages[i + 1];
+
+    if (
+      current.role === 'assistant' &&
+      current.tool_calls &&
+      next &&
+      next.role === 'tool' &&
+      next.tool_call_id === current.tool_calls[0]?.id
+    ) {
+      const nextNext = messages[i + 2];
+      const nextNextNext = messages[i + 3];
+
+      if (
+        nextNext &&
+        nextNext.role === 'assistant' &&
+        nextNext.tool_calls &&
+        nextNextNext &&
+        nextNextNext.role === 'tool' &&
+        nextNextNext.tool_call_id === nextNext.tool_calls[0]?.id &&
+        nextNext.tool_calls[0]?.name === current.tool_calls[0]?.name &&
+        nextNext.tool_calls[0]?.arguments === current.tool_calls[0]?.arguments &&
+        nextNextNext.content === next.content
+      ) {
+        result.push(current, next);
+        i += 4;
+        removedCount += 2;
+        freedTokens += Math.ceil(nextNext.content.length / 4);
+        freedTokens += Math.ceil(current.content.length / 4);
+      } else {
+        result.push(current);
+        i++;
+      }
+    } else {
+      result.push(current);
+      i++;
+    }
+  }
+
+  return { messages: result, stats: { removedCount, freedTokens } };
+}