refactor: deduplicate engine internals, replace bubble sorts, remove dead code

Extract shared code that was duplicated across functions:
- taskJSON struct (MarshalJSON/UnmarshalJSON) to package-level type
- scanTask(scanner) helper for GetTask/GetTasks (~70 identical lines)
- monthNames map for parseMonthName/parseDayAndMonth
- applyNonDateAttribute helper for Apply/ApplyToNew
- resolveDisplayID calls replace inline loops in FormatTaskListWithFormat

Replace O(n²) bubble sorts with sort.Slice in all four report sort
functions (sortByUrgency, NewestReport, NextReport, OldestReport).

Remove dead code: formatTimeWithColor (unused, also used time.Now()
instead of timeNow()), getCurrentTimestamp (unnecessary wrapper).

Remove ~20 comments that restated the next line of code.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-task/internal/engine/database.go

@@ -334,7 +334,7 @@ func runMigrations() error {
 			if _, err := tx.Exec(
 				"INSERT INTO schema_version (version, applied_at) VALUES (?, ?)",
 				migration.version,
-				getCurrentTimestamp(),
+				GetCurrentTimestamp(),
 			); err != nil {
 				tx.Rollback()
 				return fmt.Errorf("failed to record migration %d: %w", migration.version, err)
@@ -349,14 +349,9 @@ func runMigrations() error {
 	return nil
 }
 
-// getCurrentTimestamp returns the current Unix timestamp
-func getCurrentTimestamp() int64 {
-	return timeNow().Unix()
-}
-
 // GetCurrentTimestamp returns the current Unix timestamp (exported for API use)
 func GetCurrentTimestamp() int64 {
-	return getCurrentTimestamp()
+	return timeNow().Unix()
 }
 
 // CleanupChangeLog removes old change log entries based on retention policy