Plugin/hook SDK for message middleware (pre-send and post-receive transforms)

[PAMulligan]

Summary

Introduce a small plugin API that lets consumers run middleware around chat messages without forking the widget. Both client-side (widget) and server-side (worker) extension points should be supported.

Motivation

Customers want to inject context, redact PII, route to different models, log analytics events, and inject canned responses for specific intents. A plugin system makes Claudius extensible without bloating the core.

Acceptance Criteria

• Define ClaudiusPlugin interface with lifecycle hooks: onBeforeSend(message, ctx), onAfterReceive(message, ctx), onError(err, ctx)
• Hooks can be async and may modify, replace, or short-circuit messages
• ChatWidget accepts a plugins: ClaudiusPlugin[] prop
• Server-side worker exposes equivalent hooks in hono middleware form
• Three reference plugins: pluginAnalytics, pluginRedactPII, pluginCannedResponses
• Documented in the docs site with copy-paste examples

[PAMulligan]

Moved to the v2.0 milestone and sequenced behind #79: the Plugin SDK RFC should settle the hook surface (pre-send / post-receive middleware, registration, error semantics) before this implementation lands. #79 stays in Developer Experience & Distribution since the RFC itself is v1-era design work.