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>

README.md

@@ -39,12 +39,85 @@ Add to `~/.claude/settings.json` under `mcpServers`:
 | `task_show` | Show task details including checklist and dependencies |
 | `task_close` | Mark task as completed |
 | `task_update` | Update task fields |
+| `task_investigate` | Start investigation workflow (auto-links subsequent tasks) |
 | `task_similar` | Find semantically similar tasks using pgvector |
 | `task_context` | Get related tasks for current work context |
 | `task_link` | Create dependency between tasks |
 | `task_checklist_add` | Add checklist item to task |
 | `task_checklist_toggle` | Toggle checklist item |
 
+## Investigation Workflow (CF-166)
+
+The investigation workflow helps organize multi-step problem analysis by automatically linking related tasks.
+
+### Starting an Investigation
+
+```typescript
+// Start investigation
+task_investigate({
+  title: "Investigate $4,746 Brave API cost overrun",
+  project: "CF",
+  priority: "P1",
+  description: "Determine why Brave API costs are 10x expected"
+});
+```
+
+This creates an investigation task and sets it as the session's investigation parent. All subsequent tasks created in this session will automatically link to the investigation.
+
+### Auto-Linking Behavior
+
+When you create tasks during an active investigation:
+
+1. **Investigation Parent Linking**: New tasks automatically link to the investigation task
+2. **Current Task Linking**: Tasks also link to the current working task (if different from investigation)
+3. **Time-Based Linking**: If no investigation is active, tasks created within 1 hour auto-link to recent tasks in the same session
+
+### Task Types
+
+- `task`: General work item (default)
+- `bug`: Bug fix
+- `feature`: New feature
+- `debt`: Technical debt
+- `investigation`: Multi-step problem analysis (use with `task_investigate`)
+
+### Example Investigation Flow
+
+```typescript
+// 1. Start investigation
+task_investigate({
+  title: "Investigate database performance degradation",
+  project: "CF"
+});
+// Creates CF-100 (investigation task)
+
+// 2. Create subtasks - these auto-link to CF-100
+task_add({
+  title: "Check slow query log",
+  project: "CF"
+});
+// Creates CF-101, auto-links to CF-100
+
+task_add({
+  title: "Analyze connection pool metrics",
+  project: "CF"
+});
+// Creates CF-102, auto-links to CF-100
+
+// 3. View investigation with all linked tasks
+task_show({ id: "CF-100" });
+// Shows investigation with all related subtasks
+```
+
+### Session Context
+
+The MCP server tracks session context to enable smart auto-linking:
+
+- **Current Task**: The task you're actively working on (set when status changes to `in_progress`)
+- **Investigation Parent**: The investigation task all new tasks should link to
+- **Auto-link Enabled**: Whether to automatically create task links (default: true)
+
+Session context is stored per session and cleared when tasks are completed or sessions end.
+
 ## Build
 
 ```bash