feat: Add more link types to task system

New link types added:
- depends_on: Task depends on another task to complete first
- needs: Task needs another task (weaker dependency)
- implements: Task implements a feature/requirement
- fixes: Task fixes an issue identified in another task
- causes: Task causes/introduces issue in another task
- subtask_of: Task is a subtask of another (parent-child)

Preserves existing types: blocks, relates_to, duplicates

Changes:
- Migration 021 updates database constraint
- Updated TypeScript types and MCP tool schema
- Applied to production database

Implements CF-385

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

src/types.ts

@@ -36,7 +36,7 @@ export interface TaskLink {
   id: number;
   from_task_id: string;
   to_task_id: string;
-  link_type: 'blocks' | 'relates_to' | 'duplicates';
+  link_type: 'blocks' | 'relates_to' | 'duplicates' | 'depends_on' | 'needs' | 'implements' | 'fixes' | 'causes' | 'subtask_of';
   created_at: Date;
 }