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>

migrations/021_add_link_types.sql

@@ -0,0 +1,39 @@
+-- Migration 021: Add more link types to task_links table
+-- CF-385: Add more link types to task system (needs, depends_on, etc.)
+--
+-- New link types:
+--   depends_on   - Task depends on another task to complete first
+--   needs        - Task needs another task (similar to depends_on but weaker)
+--   implements   - Task implements a feature/requirement from another task
+--   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 relationship)
+--
+-- Existing types preserved:
+--   blocks       - Task blocks another task
+--   relates_to   - Generic relationship
+--   duplicates   - Task is a duplicate of another
+
+-- Drop existing constraint
+ALTER TABLE task_links DROP CONSTRAINT IF EXISTS task_links_link_type_check;
+
+-- Add new constraint with extended link types
+ALTER TABLE task_links ADD CONSTRAINT task_links_link_type_check
+  CHECK (link_type IN (
+    'blocks',
+    'relates_to',
+    'duplicates',
+    'depends_on',
+    'needs',
+    'implements',
+    'fixes',
+    'causes',
+    'subtask_of'
+  ));
+
+-- Add comments for documentation
+COMMENT ON COLUMN task_links.link_type IS 'Type of relationship between tasks: blocks, relates_to, duplicates, depends_on, needs, implements, fixes, causes, subtask_of (CF-385)';
+
+-- Record migration
+INSERT INTO schema_migrations (version, applied_at) VALUES ('021_add_link_types', NOW())
+ON CONFLICT DO NOTHING;

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'],
     },

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;
 }