Implement opal-task Phase 1: Database foundation

- Add SQLite database with schema for tasks, tags, and working_set
- Implement config management with Viper (opal.yml)
- Create Task struct with proper types (*time.Time, Priority int)
- Add database migration system
- Implement recurrence pattern parsing (1d, 1w, 1m, 1y)
- Setup project structure with cmd/ and internal/engine/
- Add dependencies: sqlite3, uuid, cobra, viper, color

opal-task/internal/engine/ws.go

@@ -0,0 +1,9 @@
+package engine
+
+// WorkingSet is a mapping from small integers to task uuids for all pending tasks.
+// The small integers are meant to be stable, easily-typed identifiers for users to interact with
+// important tasks.
+type WorkingSet struct {
+	byUUID map[string]*Task
+	byID   []string
+}