fix: clear golangci-lint backlog and make lint job blocking

Addresses errcheck, gosec, revive, staticcheck, and unused findings
across the codebase (unchecked error returns, unsafe file inclusion
warnings on operator/test-controlled paths, missing package comments,
unused parameters, deprecated API usage). Also fixes two suppression
comments that were silently no-ops due to wrong syntax (#nosec needs
a leading '#', nolint reasons need '//' not '--').

With the backlog clear, drop continue-on-error from the CI lint job
per the plan left in b4b4806.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/domain/notification.go

@@ -1,3 +1,6 @@
+// Package domain contains the core types shared across the notifier
+// service - notifications, queueing primitives, and the notifier
+// interfaces that provider implementations satisfy.
 package domain
 
 import (
@@ -15,6 +18,7 @@ var (
 // Priority defines the urgency level of a notification
 type Priority int
 
+// Priority levels, in increasing order of urgency.
 const (
 	PriorityLow Priority = iota
 	PriorityNormal
@@ -25,6 +29,7 @@ const (
 // NotificationType defines the channel through which to send the notification
 type NotificationType string
 
+// Supported notification channels.
 const (
 	TypeEmail  NotificationType = "email"
 	TypeSlack  NotificationType = "slack"
@@ -35,6 +40,7 @@ const (
 // ContentType defines the format of the notification body
 type ContentType string
 
+// Supported body content types.
 const (
 	ContentTypeText ContentType = "text"
 	ContentTypeHTML ContentType = "html"
@@ -43,6 +49,7 @@ const (
 // NotificationStatus represents the current state of a notification
 type NotificationStatus string
 
+// Notification lifecycle states.
 const (
 	StatusPending    NotificationStatus = "pending"
 	StatusQueued     NotificationStatus = "queued"