feat(messages): include reactions in JSON output

[piekstra]

Closes #113

Why

The Slack API returns reaction data on messages from conversations.replies and conversations.history, but the CLI's Message struct had no Reactions field — so this data was silently discarded during JSON unmarshalling. Integrations that consume -o json output couldn't see which reactions were on messages, losing important thread context (e.g., acknowledgements, approvals).

What changed

• Added Reaction type (name, count, users) to the client package
• Added Reactions []Reaction field (with omitempty) to Message struct
• The Slack API already returns this data — no API call changes needed, just struct alignment

Test plan

• New client tests verify reactions are deserialized from mock API responses (both GetThreadReplies and GetChannelHistory)
• New command tests verify reactions appear in JSON output from runThread and runHistory
• Existing tests continue to pass (messages without reactions are unaffected due to omitempty)
• Full test suite green