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/tools/index.ts

@@ -133,7 +133,7 @@ export const toolDefinitions = [
       properties: {
         from_id: { type: 'string', description: 'Source task ID' },
         to_id: { type: 'string', description: 'Target task ID' },
-        link_type: { type: 'string', enum: ['blocks', 'relates_to', 'duplicates'], description: 'Relationship type' },
+        link_type: { type: 'string', enum: ['blocks', 'relates_to', 'duplicates', 'depends_on', 'needs', 'implements', 'fixes', 'causes', 'subtask_of'], description: 'Relationship type' },
       },
       required: ['from_id', 'to_id', 'link_type'],
     },