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/purge-errors.ts

@@ -0,0 +1,34 @@
+// Purge-errors strategy — removes failed/empty tool_result entries.
+// Clean-room implementation.
+
+import type { DcpMessage } from '../messages.js';
+
+const ERROR_PREFIXES = ['Error:', 'error:', 'Error: '];
+const DEFAULT_WINDOW = 5;
+
+export function purgeErrors(
+  messages: DcpMessage[],
+  windowSize: number = DEFAULT_WINDOW,
+): { messages: DcpMessage[]; stats: { removedCount: number; freedTokens: number } } {
+  const result: DcpMessage[] = [];
+  let removedCount = 0;
+  let freedTokens = 0;
+
+  for (const msg of messages) {
+    if (msg.role === 'tool') {
+      const shouldRemove =
+        msg.isError ||
+        ERROR_PREFIXES.some((p) => msg.content.startsWith(p)) ||
+        msg.content.trim() === '';
+
+      if (shouldRemove) {
+        removedCount++;
+        freedTokens += Math.ceil(msg.content.length / 4);
+        continue; // Skip this message
+      }
+    }
+    result.push(msg);
+  }
+
+  return { messages: result, stats: { removedCount, freedTokens } };
+}