feat(plugin-sdk): Add channelRuntime support for external channel plugins

## Overview

This PR enables external channel plugins (loaded via Plugin SDK) to access
advanced runtime features like AI response dispatching, which were previously
only available to built-in channels.

## Changes

### src/gateway/server-channels.ts
- Import PluginRuntime type
- Add optional channelRuntime parameter to ChannelManagerOptions
- Pass channelRuntime to channel startAccount calls via conditional spread
- Ensures backward compatibility (field is optional)

### src/gateway/server.impl.ts
- Import createPluginRuntime from plugins/runtime
- Create and pass channelRuntime to channel manager

### src/channels/plugins/types.adapters.ts
- Import PluginRuntime type
- Add comprehensive documentation for channelRuntime field
- Document available features, use cases, and examples
- Improve type safety (use imported PluginRuntime type vs inline import)

## Benefits

External channel plugins can now:
- Generate AI-powered responses using dispatchReplyWithBufferedBlockDispatcher
- Access routing, text processing, and session management utilities
- Use command authorization and group policy resolution
- Maintain feature parity with built-in channels

## Backward Compatibility

- channelRuntime field is optional in ChannelGatewayContext
- Conditional spread ensures it's only passed when explicitly provided
- Existing channels without channelRuntime support continue to work unchanged
- No breaking changes to channel plugin API

## Testing

- Email channel plugin successfully uses channelRuntime for AI responses
- All existing built-in channels (slack, discord, telegram, etc.) work unchanged
- Gateway loads and runs without errors when channelRuntime is provided

src/gateway/server.impl.ts

@@ -46,6 +46,7 @@ import { startDiagnosticHeartbeat, stopDiagnosticHeartbeat } from "../logging/di
 import { createSubsystemLogger, runtimeForLogger } from "../logging/subsystem.js";
 import { getGlobalHookRunner, runGlobalGatewayStopSafely } from "../plugins/hook-runner-global.js";
 import { createEmptyPluginRegistry } from "../plugins/registry.js";
+import { createPluginRuntime } from "../plugins/runtime/index.js";
 import type { PluginServicesHandle } from "../plugins/services.js";
 import { getTotalQueueSize } from "../process/command-queue.js";
 import type { RuntimeEnv } from "../runtime.js";
@@ -554,6 +555,7 @@ export async function startGatewayServer(
     loadConfig,
     channelLogs,
     channelRuntimeEnvs,
+    channelRuntime: createPluginRuntime().channel,
   });
   const { getRuntimeSnapshot, startChannels, startChannel, stopChannel, markChannelLoggedOut } =
     channelManager;