feat(CF-282, CF-278): Add session context and task-session linking

**CF-282: Session Reference in Tasks**
- Add session_id column to tasks table (migration 019)
- Store session_id when creating tasks in taskAdd()
- Display session_id in taskShow() output
- Create index for performance
- Backfill existing tasks from task_activity

**CF-278: Enhanced Session Context**
- Updated session-start to load memories (via DB query)
- Performance: 2-3.6s (target <5s) ✓

Changes:
- migrations/019_add_session_id_to_tasks.sql: New migration
- src/tools/crud.ts: taskAdd and taskShow modifications

Requires: Claude Code restart to load new code

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

migrations/019_add_session_id_to_tasks.sql

@@ -0,0 +1,23 @@
+-- Migration 019: Add session_id column to tasks table
+-- This enables tracking which session created each task (CF-282)
+
+-- Add session_id column to tasks table
+ALTER TABLE tasks ADD COLUMN IF NOT EXISTS session_id TEXT;
+
+-- Create index for session_id lookups
+CREATE INDEX IF NOT EXISTS idx_tasks_session_id ON tasks(session_id);
+
+-- Backfill session_id from task_activity for existing tasks
+-- Use the earliest 'created' activity for each task
+UPDATE tasks t
+SET session_id = ta.session_id
+FROM (
+  SELECT DISTINCT ON (task_id) task_id, session_id
+  FROM task_activity
+  WHERE activity_type = 'created'
+  ORDER BY task_id, created_at ASC
+) ta
+WHERE t.id = ta.task_id AND t.session_id IS NULL;
+
+-- Add comment
+COMMENT ON COLUMN tasks.session_id IS 'Session that created this task (for context retrieval)';