feat(mattermost): add ws and zod dependencies
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Mattermost plugin: ship runtime deps for zod/ws
Optional body (if you want one):
Fixes plugin load failure when installed under ~/.clawdbot/extensions by declaring zod and ws as runtime dependencies.
Move mattermost channel implementation from core to extensions/mattermost plugin. Extract config schema, group mentions, normalize utilities, and all mattermost-specific logic (accounts, client, monitor, probe, send) into the extension. Update imports to use plugin SDK and local modules. Add channel metadata directly in plugin definition instead of using getChatChannelMeta. Update package.json with channel and install configuration.
Add Mattermost as a supported messaging channel with bot API and WebSocket integration. Includes channel state tracking (tint, summary, details), multi-account support, and delivery target routing. Update documentation and tests to include Mattermost alongside existing channels.
