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/index.ts

@@ -37,7 +37,7 @@ import {
 
 import { testConnection, close } from './db.js';
 import { toolDefinitions } from './tools/index.js';
-import { taskAdd, taskList, taskShow, taskClose, taskUpdate } from './tools/crud.js';
+import { taskAdd, taskList, taskShow, taskClose, taskUpdate, taskInvestigate } from './tools/crud.js';
 import { taskSimilar, taskContext } from './tools/search.js';
 import { taskLink, checklistAdd, checklistToggle, taskResolveDuplicate } from './tools/relations.js';
 import { epicAdd, epicList, epicShow, epicAssign, epicClose } from './tools/epics.js';
@@ -139,6 +139,14 @@ server.setRequestHandler(CallToolRequestSchema, async (request) => {
           title: a.title,
         });
         break;
+      case 'task_investigate':
+        result = await taskInvestigate({
+          title: a.title,
+          project: a.project,
+          priority: a.priority,
+          description: a.description,
+        });
+        break;
 
       // Search
       case 'task_similar':