feat(CF-450): Check completed tasks on task creation to avoid circular work

Enhanced task_add to search both open AND completed tasks when checking for
duplicates. Previously only checked non-completed tasks.

**Changes:**
- Removed `status != 'completed'` filter from similarity search
- Increased search limit from 3 to 5 tasks
- Separated display: open tasks vs previously completed
- Show 150-char context snippet from completed task descriptions
- Suggest using "task show <id>" to see full solution

**Benefits:**
- Prevents recreating already-solved tasks
- Surfaces solutions from previous attempts
- Reduces circular work on recurring issues
- Maintains context from closed tasks

**Example output:**
```
⚠️  Similar tasks found:

**Open/In Progress:**
  - CF-442: Next.js cache issue (92% match, open)

**Previously Completed:**
  - CF-378: Pre-test validation (85% match)
    Context: "Created comprehensive pre-test quality validation workflow..."

  💡 Use "task show <id>" to see full solution before recreating work
```

Fixes: CF-450

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/tools/crud.ts

@@ -95,14 +95,15 @@ export async function taskAdd(args: TaskAddArgs): Promise<string> {
   const embeddingValue = embedding ? formatEmbedding(embedding) : null;
 
   // Check for similar/duplicate tasks (only if embedding succeeded)
+  // CF-450: Check both open AND completed tasks to avoid circular work
   let duplicateWarning = '';
   if (embeddingValue) {
-    const similarTasks = await query<{ id: string; title: string; status: string; similarity: number }>(
-      `SELECT id, title, status, 1 - (embedding <=> $1) as similarity
+    const similarTasks = await query<{ id: string; title: string; status: string; description: string; similarity: number }>(
+      `SELECT id, title, status, description, 1 - (embedding <=> $1) as similarity
        FROM tasks
-       WHERE project = $2 AND embedding IS NOT NULL AND status != 'completed'
+       WHERE project = $2 AND embedding IS NOT NULL
        ORDER BY embedding <=> $1
-       LIMIT 3`,
+       LIMIT 5`,
       [embeddingValue, projectKey]
     );
 
@@ -110,11 +111,35 @@ export async function taskAdd(args: TaskAddArgs): Promise<string> {
     const highSimilarity = similarTasks.filter(t => t.similarity > 0.70);
     if (highSimilarity.length > 0) {
       duplicateWarning = '\n\n⚠️  Similar tasks found:\n';
-      for (const t of highSimilarity) {
+
+      const openTasks = highSimilarity.filter(t => t.status !== 'completed');
+      const completedTasks = highSimilarity.filter(t => t.status === 'completed');
+
+      if (openTasks.length > 0) {
+        duplicateWarning += '\n**Open/In Progress:**\n';
+        for (const t of openTasks) {
           const pct = Math.round(t.similarity * 100);
           duplicateWarning += `  - ${t.id}: ${t.title} (${pct}% match, ${t.status})\n`;
         }
-      duplicateWarning += '\nConsider linking with: task link <from> <to> relates_to';
+      }
+
+      if (completedTasks.length > 0) {
+        duplicateWarning += '\n**Previously Completed:**\n';
+        for (const t of completedTasks) {
+          const pct = Math.round(t.similarity * 100);
+          duplicateWarning += `  - ${t.id}: ${t.title} (${pct}% match)\n`;
+
+          // Show snippet of solution/outcome from description
+          if (t.description) {
+            const snippet = t.description.substring(0, 150).replace(/\n/g, ' ').replace(/"/g, '\\"');
+            const ellipsis = t.description.length > 150 ? '...' : '';
+            duplicateWarning += `    Context: "${snippet}${ellipsis}"\n`;
+          }
+        }
+        duplicateWarning += '\n  💡 Use "task show <id>" to see full solution before recreating work\n';
+      }
+
+      duplicateWarning += '\nConsider linking with: task link <new-id> <related-id> relates_to';
     }
   }