critical: Fix SSH timeout + resumption logic bugs

**SSH Command Fix:**
- CRITICAL: Removed && after background command (&)
- Pattern: 'cmd & echo Started' works, 'cmd && echo' waits forever
- Manually tested: Works perfectly on direct SSH
- Result: Chunk 0 now starts successfully on worker1 (24 processes running)

**Resumption Logic Fix:**
- CRITICAL: Only count completed/running chunks, not pending
- Query: Added 'AND status IN (completed, running)' filter
- Result: Starts from chunk 0 when no chunks complete (was skipping to chunk 3)

**Database Cleanup:**
- CRITICAL: Delete pending/failed chunks on coordinator start
- Prevents UNIQUE constraint errors on retry
- Result: Clean slate allows coordinator to assign chunks fresh

**Verification:**
- ✅ Chunk v9_chunk_000000: status='running', assigned_worker='worker1'
- ✅ Worker1: 24 Python processes running backtester
- ✅ Database: Cleaned 3 pending chunks, created 1 running chunk
- ⚠️  Worker2: SSH hop still timing out (separate infrastructure issue)

Files changed:
- cluster/distributed_coordinator.py (3 critical fixes: line 388-401, 514-533, 507-514)

cluster/distributed_coordinator.py

@@ -388,11 +388,17 @@ class DistributedCoordinator:
         ssh_opts = "-o StrictHostKeyChecking=no -o ConnectTimeout=10 -o ServerAliveInterval=5"
         
         # Execute command and capture result to verify it started
+        # CRITICAL FIX (Dec 1, 2025): Remove && because command ends with & (background)
+        # In single quotes, & backgrounds process and shell continues to next command (echo)
         if 'ssh_hop' in worker:
             # Worker 2 requires hop through worker 1
-            ssh_cmd = f"ssh {ssh_opts} {worker['ssh_hop']} \"ssh {ssh_opts} {worker['host']} '{cmd}' && echo 'Started chunk {chunk_id}' || echo 'FAILED'\""
+            ssh_cmd = f"ssh {ssh_opts} {worker['ssh_hop']} \"ssh {ssh_opts} {worker['host']} '{cmd} echo Started_chunk_{chunk_id}'\""
         else:
-            ssh_cmd = f"ssh {ssh_opts} {worker['host']} '{cmd}' && echo 'Started chunk {chunk_id}' || echo 'FAILED'"
+            ssh_cmd = f"ssh {ssh_opts} {worker['host']} '{cmd} echo Started_chunk_{chunk_id}'"
+        
+        # DEBUG: Print command for troubleshooting
+        print(f"   DEBUG: Executing SSH command:")
+        print(f"   {ssh_cmd[:200]}...")  # First 200 chars
         
         # Use run() to capture output and verify success
         # CRITICAL FIX (Dec 1, 2025): Increase timeout from 30s to 60s for nested SSH hops
@@ -406,7 +412,8 @@ class DistributedCoordinator:
             )
             
             # Verify worker process started
-            if 'Started chunk' in result.stdout:
+            # CRITICAL FIX (Dec 1, 2025): Look for marker without spaces (Started_chunk_)
+            if f'Started_chunk_{chunk_id}' in result.stdout:
                 print(f"✅ Chunk {chunk_id} started on {worker_id} successfully")
                 return True
             else:
@@ -503,26 +510,38 @@ class DistributedCoordinator:
         print("🔄 Distributing chunks to workers...")
         print()
         
-        # CRITICAL FIX (Nov 30, 2025): Resume from existing chunks in database
-        # Get max chunk ID to avoid UNIQUE constraint errors on restart
+        # CRITICAL FIX (Dec 1, 2025): Clear pending/failed chunks to allow retry
+        # This prevents UNIQUE constraint errors on restart
         conn = sqlite3.connect(self.db.db_path)
         c = conn.cursor()
-        c.execute("SELECT id FROM chunks WHERE id LIKE 'v9_chunk_%' ORDER BY id DESC LIMIT 1")
+        c.execute("DELETE FROM chunks WHERE status IN ('pending', 'failed')")
+        deleted_count = c.rowcount
+        conn.commit()
+        conn.close()
+        if deleted_count > 0:
+            print(f"🗑️  Cleared {deleted_count} pending/failed chunks for retry")
+            print()
+        
+        # CRITICAL FIX (Dec 1, 2025): Resume from existing COMPLETED chunks only
+        # Get max chunk ID from completed/running chunks, not all chunks
+        # This allows retrying failed/pending chunks without UNIQUE constraint errors
+        conn = sqlite3.connect(self.db.db_path)
+        c = conn.cursor()
+        c.execute("SELECT id FROM chunks WHERE id LIKE 'v9_chunk_%' AND status IN ('completed', 'running') ORDER BY id DESC LIMIT 1")
         last_chunk = c.fetchone()
         conn.close()
         
         if last_chunk:
-            # Extract counter from last chunk ID (e.g., "v9_chunk_000042" -> 42)
+            # Extract counter from last completed chunk ID
             last_counter = int(last_chunk[0].split('_')[-1])
             chunk_id_counter = last_counter + 1
-            # Resume from where we left off in the parameter space
             chunk_start = chunk_id_counter * chunk_size
-            print(f"📋 Resuming from chunk {chunk_id_counter} (found {last_counter + 1} existing chunks)")
+            print(f"📋 Resuming from chunk {chunk_id_counter} (found {last_counter + 1} completed chunks)")
             print(f"   Starting at combo {chunk_start:,} / {total_combos:,}")
         else:
             chunk_id_counter = 0
             chunk_start = 0
-            print(f"📋 Starting fresh - no existing chunks found")
+            print(f"📋 Starting fresh - no completed chunks found")
         
         # Split work across workers
         active_chunks = {}