ax(mining): replace banned fmt import with MiningError in container.go

container.go used fmt.Errorf for all error construction, violating the
AX banned-imports rule. Replaced with ErrInternal().WithCause() using
the package's own error primitives, consistent with the rest of pkg/mining.

Co-Authored-By: Charon <charon@lethean.io>

pkg/mining/container.go

@@ -2,7 +2,6 @@ package mining
 
 import (
 	"context"
-	"fmt"
 	"sync"
 
 	"forge.lthn.ai/Snider/Mining/pkg/database"
@@ -83,13 +82,13 @@ func (c *Container) Initialize(ctx context.Context) error {
 	defer c.mutex.Unlock()
 
 	if c.initialized {
-		return fmt.Errorf("container already initialized")
+		return ErrInternal("container already initialized")
 	}
 
 	// 1. Initialize database (optional)
 	if c.config.Database.Enabled {
 		if err := database.Initialize(c.config.Database); err != nil {
-			return fmt.Errorf("failed to initialize database: %w", err)
+			return ErrInternal("failed to initialize database").WithCause(err)
 		}
 		c.hashrateStore = database.DefaultStore()
 		logging.Info("database initialized", logging.Fields{"retention_days": c.config.Database.RetentionDays})
@@ -102,7 +101,7 @@ func (c *Container) Initialize(ctx context.Context) error {
 	var err error
 	c.profileManager, err = NewProfileManager()
 	if err != nil {
-		return fmt.Errorf("failed to initialize profile manager: %w", err)
+		return ErrInternal("failed to initialize profile manager").WithCause(err)
 	}
 
 	// 3. Initialize miner manager
@@ -138,7 +137,7 @@ func (c *Container) Start(ctx context.Context) error {
 	defer c.mutex.RUnlock()
 
 	if !c.initialized {
-		return fmt.Errorf("container not initialized")
+		return ErrInternal("container not initialized")
 	}
 
 	// Start event hub
@@ -178,7 +177,7 @@ func (c *Container) Shutdown(ctx context.Context) error {
 	// 2. Stop node transport (only if it was started)
 	if c.nodeService != nil && c.transportStarted {
 		if err := c.nodeService.StopTransport(); err != nil {
-			errs = append(errs, fmt.Errorf("node transport: %w", err))
+			errs = append(errs, ErrInternal("node transport shutdown failed").WithCause(err))
 		}
 		c.transportStarted = false
 	}
@@ -195,14 +194,14 @@ func (c *Container) Shutdown(ctx context.Context) error {
 
 	// 5. Close database
 	if err := database.Close(); err != nil {
-		errs = append(errs, fmt.Errorf("database: %w", err))
+		errs = append(errs, ErrInternal("database shutdown failed").WithCause(err))
 	}
 
 	c.initialized = false
 	close(c.shutdownCh)
 
 	if len(errs) > 0 {
-		return fmt.Errorf("shutdown errors: %v", errs)
+		return ErrInternal("shutdown completed with errors").WithCause(errs[0])
 	}
 
 	logging.Info("service container shutdown complete", nil)