feat(container): integrate plugin interception into container resolution

[markshust]

Summary

• Wires PluginInterceptor into Container::resolve() so plugins actually intercept method calls on resolved objects — previously plugins were discovered and registered but never applied
• Adds setPluginInterceptor() setter on Container (avoids circular dependency with PluginInterceptor needing ContainerInterface)
• Wraps resolved instances with PluginProxy in both autowiring and closure binding code paths; singleton caching stores the proxy
• Pre-registered instances via instance() are excluded from wrapping (explicit over implicit)
• Adds PluginProxy::getPluginTarget() so reflection-based code (like Router) can inspect the real object
• Router unwraps PluginProxy before reflecting on controller method parameters
• Prevents plugins from targeting other plugin classes with a loud PluginException suggesting Preferences instead
• 22 new tests covering container integration, end-to-end interception, and plugin-on-plugin validation

Test plan

• All 4985 existing + new tests pass in parallel
• Verified plugin discovery, proxy wrapping, and interception in MarkoTalk app
• Confirmed X-Plugin-Intercepted header appears on proxied controller response
• Verify MarkoTalk's MarkdownPlugin and MentionExtractorPlugin fire during message save

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