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/types.ts

@@ -5,7 +5,7 @@ export interface Task {
   project: string;
   title: string;
   description?: string;
-  type: 'task' | 'bug' | 'feature' | 'debt';
+  type: 'task' | 'bug' | 'feature' | 'debt' | 'investigation';
   status: 'open' | 'in_progress' | 'testing' | 'blocked' | 'completed';
   priority: 'P0' | 'P1' | 'P2' | 'P3';
   version_id?: string;