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/index.ts

@@ -25,7 +25,7 @@ export const toolDefinitions = [
       properties: {
         project: { type: 'string', description: 'Filter by project key' },
         status: { type: 'string', enum: ['open', 'in_progress', 'testing', 'blocked', 'completed'], description: 'Filter by status' },
-        type: { type: 'string', enum: ['task', 'bug', 'feature', 'debt'], description: 'Filter by type' },
+        type: { type: 'string', enum: ['task', 'bug', 'feature', 'debt', 'investigation'], description: 'Filter by type' },
         priority: { type: 'string', enum: ['P0', 'P1', 'P2', 'P3'], description: 'Filter by priority' },
         limit: { type: 'number', description: 'Max results (default: 20)' },
       },
@@ -62,12 +62,26 @@ export const toolDefinitions = [
         id: { type: 'string', description: 'Task ID to update' },
         status: { type: 'string', enum: ['open', 'in_progress', 'testing', 'blocked', 'completed'], description: 'New status' },
         priority: { type: 'string', enum: ['P0', 'P1', 'P2', 'P3'], description: 'New priority' },
-        type: { type: 'string', enum: ['task', 'bug', 'feature', 'debt'], description: 'New type' },
+        type: { type: 'string', enum: ['task', 'bug', 'feature', 'debt', 'investigation'], description: 'New type' },
         title: { type: 'string', description: 'New title' },
       },
       required: ['id'],
     },
   },
+  {
+    name: 'task_investigate',
+    description: 'Start an investigation workflow: creates an investigation task and auto-links all subsequent tasks to it. Use when beginning multi-step problem analysis.',
+    inputSchema: {
+      type: 'object',
+      properties: {
+        title: { type: 'string', description: 'Investigation title (required)' },
+        project: { type: 'string', description: 'Project key (e.g., ST, VPN). Auto-detected from CWD if not provided.' },
+        priority: { type: 'string', enum: ['P0', 'P1', 'P2', 'P3'], description: 'Priority level (default: P1)' },
+        description: { type: 'string', description: 'Optional description of investigation scope' },
+      },
+      required: ['title'],
+    },
+  },
 
   // Semantic Search Tools
   {