Message threading support

[intendednull]

Summary

Add first-class message threading as a general feature. Currently replies use Reply { parent, body } content to link messages, but there's no dedicated thread UI or thread-scoped context.

A proper threading system would:

• Allow users to open a thread view from any message
• Show threaded replies in a side panel or inline expansion
• Track unread counts per thread
• Let users follow/mute individual threads

This came up during the LLM agent UX design (docs/design/llm-agent-ux-spec.md) — agents benefit from thread-scoped context, but threading is useful for all users in busy channels.

Motivation

• Keeps channel-level chat clean when side conversations happen
• Provides natural context boundaries for agent interactions
• Matches user expectations from Discord/Slack thread UX

[intendednull]

Message threading has been implemented. EventKind::Message includes a reply_to: Option<EventHash> field at crates/state/src/event.rs:131-137 and crates/state/src/types.rs:76. This enables messages to reference parent messages by their content hash, providing threading support at the protocol level.

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