feat(jobs): wire mcp_tokens_purge runner into jobs.Module

- Config.MCPTokensPurge + scheduleEntry branch
- app.go constructs runner with mcpTokensPurgeAdapter
- Registered as 'mcp_tokens_purge' in RunnerFns map

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

internal/jobs/domain.go

@@ -127,6 +127,13 @@ type AcpEventsPurgeConfig struct {
 	Retention time.Duration
 }
 
+// MCPTokensPurgeConfig extends RunnerConfig with mcp_tokens retention.
+type MCPTokensPurgeConfig struct {
+	Enabled   bool
+	Interval  time.Duration
+	Retention time.Duration
+}
+
 // Config is the full jobs.Module configuration.
 type Config struct {
 	Enabled           bool
@@ -138,4 +145,5 @@ type Config struct {
 	JobReaper         JobReaperConfig
 	JobPurge          JobPurgeConfig
 	AcpEventsPurge    AcpEventsPurgeConfig
+	MCPTokensPurge    MCPTokensPurgeConfig
 }