feat(CF-166): Add investigation workflow with auto-linking

Implements comprehensive investigation task management:

**Schema Changes (Migration 020):**
- Add 'investigation' task type to tasks table
- Add investigation_parent_id and auto_link_enabled columns to session_context
- Add auto_linked and linked_at columns to task_links for tracking
- Create indexes for efficient auto-linking queries

**Auto-Linking Logic:**
1. Investigation Parent Linking: Tasks auto-link to active investigation
2. Current Task Linking: Tasks link to current working task
3. Time-Based Linking: Tasks within 1 hour auto-link (fallback)

**New Tool:**
- task_investigate: Start investigation workflow, sets session context

**Documentation:**
- Add comprehensive investigation workflow guide to README
- Include examples and session context explanation

All checklist items completed:
✓ Investigation task type in schema
✓ Session context table enhancements
✓ Auto-linking implementation
✓ task_investigate command
✓ Documentation updates

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

src/tools/crud.ts

@@ -142,25 +142,75 @@ export async function taskAdd(args: TaskAddArgs): Promise<string> {
   // Record activity for session tracking
   await recordActivity(taskId, 'created', undefined, 'open');
 
-  // Check for session context and auto-link to current working task
+  // Enhanced auto-linking logic (CF-166)
   let autoLinkMessage = '';
   try {
-    const sessionContext = await queryOne<{ current_task_id: string }>(
-      `SELECT current_task_id FROM session_context WHERE session_id = $1`,
+    const sessionContext = await queryOne<{
+      current_task_id: string | null;
+      investigation_parent_id: string | null;
+      auto_link_enabled: boolean;
+    }>(
+      `SELECT current_task_id, investigation_parent_id, auto_link_enabled
+       FROM session_context WHERE session_id = $1`,
       [session_id]
     );
 
-    if (sessionContext?.current_task_id) {
-      // Auto-link to current working task
-      await taskLink({
-        from_id: taskId,
-        to_id: sessionContext.current_task_id,
-        link_type: 'relates_to'
-      });
-      autoLinkMessage = `\n\n🔗 Auto-linked to: ${sessionContext.current_task_id} (current working task)`;
+    if (sessionContext?.auto_link_enabled !== false) {
+      const linkedTasks: string[] = [];
+
+      // 1. Auto-link to investigation parent if this is created during an investigation
+      if (sessionContext?.investigation_parent_id) {
+        await execute(
+          `INSERT INTO task_links (from_task_id, to_task_id, link_type, auto_linked)
+           VALUES ($1, $2, 'relates_to', true)
+           ON CONFLICT DO NOTHING`,
+          [taskId, sessionContext.investigation_parent_id]
+        );
+        linkedTasks.push(`${sessionContext.investigation_parent_id} (investigation)`);
+      }
+
+      // 2. Auto-link to current working task if different from investigation parent
+      if (sessionContext?.current_task_id &&
+          sessionContext.current_task_id !== sessionContext?.investigation_parent_id) {
+        await execute(
+          `INSERT INTO task_links (from_task_id, to_task_id, link_type, auto_linked)
+           VALUES ($1, $2, 'relates_to', true)
+           ON CONFLICT DO NOTHING`,
+          [taskId, sessionContext.current_task_id]
+        );
+        linkedTasks.push(`${sessionContext.current_task_id} (current task)`);
+      }
+
+      // 3. Time-based auto-linking: find tasks created within 1 hour in same session
+      if (!sessionContext?.investigation_parent_id && !sessionContext?.current_task_id) {
+        const recentTasks = await query<{ id: string; title: string }>(
+          `SELECT id, title FROM tasks
+           WHERE session_id = $1 AND id != $2
+           AND created_at > NOW() - INTERVAL '1 hour'
+           AND status != 'completed'
+           ORDER BY created_at DESC
+           LIMIT 3`,
+          [session_id, taskId]
+        );
+
+        for (const task of recentTasks) {
+          await execute(
+            `INSERT INTO task_links (from_task_id, to_task_id, link_type, auto_linked)
+             VALUES ($1, $2, 'relates_to', true)
+             ON CONFLICT DO NOTHING`,
+            [taskId, task.id]
+          );
+          linkedTasks.push(`${task.id} (recent)`);
+        }
+      }
+
+      if (linkedTasks.length > 0) {
+        autoLinkMessage = `\n\n🔗 Auto-linked to: ${linkedTasks.join(', ')}`;
+      }
     }
-  } catch {
-    // Silently fail if session context unavailable
+  } catch (error) {
+    // Log but don't fail if auto-linking fails
+    console.error('Auto-linking failed:', error);
   }
 
   return `Created: ${taskId}\n  Title: ${title}\n  Type: ${type}\n  Priority: ${priority}\n  Project: ${projectKey}${embedding ? '\n  (embedded for semantic search)' : ''}${duplicateWarning}${autoLinkMessage}`;
@@ -460,3 +510,44 @@ export async function taskUpdate(args: TaskUpdateArgs): Promise<string> {
 
   return `Updated: ${id}`;
 }
+
+/**
+ * Start an investigation workflow (CF-166)
+ * Creates an investigation task and sets it as the session context parent
+ * All subsequent tasks will auto-link to this investigation
+ */
+export async function taskInvestigate(args: TaskAddArgs): Promise<string> {
+  const { title, project, priority = 'P1', description = '' } = args;
+
+  // Create investigation task
+  const taskResult = await taskAdd({
+    title,
+    project,
+    type: 'investigation',
+    priority,
+    description: description || 'Investigation task to coordinate related subtasks',
+  });
+
+  // Extract task ID from result (format: "Created: XX-123\n...")
+  const taskIdMatch = taskResult.match(/Created: ([\w-]+)/);
+  if (!taskIdMatch) {
+    return taskResult; // Return original message if format unexpected
+  }
+  const taskId = taskIdMatch[1];
+
+  // Set as investigation parent in session context
+  const session_id = getSessionId();
+  try {
+    await execute(
+      `INSERT INTO session_context (session_id, current_task_id, investigation_parent_id)
+       VALUES ($1, $2, $2)
+       ON CONFLICT (session_id) DO UPDATE
+       SET investigation_parent_id = $2, current_task_id = $2, updated_at = NOW()`,
+      [session_id, taskId]
+    );
+  } catch (error) {
+    console.error('Failed to set investigation context:', error);
+  }
+
+  return taskResult + '\n\n🔍 Investigation started! All new tasks will auto-link to this investigation.';
+}