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/channels/plugins/types.adapters.ts

@@ -3,6 +3,7 @@ import type { OpenClawConfig } from "../../config/config.js";
 import type { GroupToolPolicyConfig } from "../../config/types.tools.js";
 import type { OutboundDeliveryResult, OutboundSendDeps } from "../../infra/outbound/deliver.js";
 import type { OutboundIdentity } from "../../infra/outbound/identity.js";
+import type { PluginRuntime } from "../../plugins/runtime/types.js";
 import type { RuntimeEnv } from "../../runtime.js";
 import type {
   ChannelAccountSnapshot,
@@ -172,6 +173,68 @@ export type ChannelGatewayContext<ResolvedAccount = unknown> = {
   log?: ChannelLogSink;
   getStatus: () => ChannelAccountSnapshot;
   setStatus: (next: ChannelAccountSnapshot) => void;
+  /**
+   * Optional channel runtime helpers for external channel plugins.
+   *
+   * This field provides access to advanced Plugin SDK features that are
+   * available to external plugins but not to built-in channels (which can
+   * directly import internal modules).
+   *
+   * ## Available Features
+   *
+   * - **reply**: AI response dispatching, formatting, and delivery
+   * - **routing**: Agent route resolution and matching
+   * - **text**: Text chunking, markdown processing, and control command detection
+   * - **session**: Session management and metadata tracking
+   * - **media**: Remote media fetching and buffer saving
+   * - **commands**: Command authorization and control command handling
+   * - **groups**: Group policy resolution and mention requirements
+   * - **pairing**: Channel pairing and allow-from management
+   *
+   * ## Use Cases
+   *
+   * External channel plugins (e.g., email, SMS, custom integrations) that need:
+   * - AI-powered response generation and delivery
+   * - Advanced text processing and formatting
+   * - Session tracking and management
+   * - Agent routing and policy resolution
+   *
+   * ## Example
+   *
+   * ```typescript
+   * const emailGatewayAdapter: ChannelGatewayAdapter<EmailAccount> = {
+   *   startAccount: async (ctx) => {
+   *     // Check availability (for backward compatibility)
+   *     if (!ctx.channelRuntime) {
+   *       ctx.log?.warn?.("channelRuntime not available - skipping AI features");
+   *       return;
+   *     }
+   *
+   *     // Use AI dispatch
+   *     await ctx.channelRuntime.reply.dispatchReplyWithBufferedBlockDispatcher({
+   *       ctx: { ... },
+   *       cfg: ctx.cfg,
+   *       dispatcherOptions: {
+   *         deliver: async (payload) => {
+   *           // Send reply via email
+   *         },
+   *       },
+   *     });
+   *   },
+   * };
+   * ```
+   *
+   * ## Backward Compatibility
+   *
+   * - This field is **optional** - channels that don't need it can ignore it
+   * - Built-in channels (slack, discord, etc.) typically don't use this field
+   *   because they can directly import internal modules
+   * - External plugins should check for undefined before using
+   *
+   * @since Plugin SDK 2026.2.19
+   * @see {@link https://docs.openclaw.ai/plugins/developing-plugins | Plugin SDK documentation}
+   */
+  channelRuntime?: PluginRuntime["channel"];
 };
 
 export type ChannelLogoutResult = {