Resolve user mentions in message text to display names

Problem

When reading messages via conversations_history, conversations_search_messages, or conversations_replies, user mentions embedded in message text remain as raw Slack IDs (e.g., <@U0AFE8TJQ0L>) instead of being resolved to human-readable names.

Example output from conversations_history:

Text
"U0A0FUA8ZRQ U09DFRLDJ7Q U02UDJPB78T U02FVMBKEP7 STACKKKED lineup today"

The message author gets resolved (via UserName and RealName columns), but mentions within the text don't.

Expected behavior

Mentions in message text should be resolved to names:

"@Katherine @Kyle @Tom @Sarah STACKKKED lineup today"

Current workaround

Call users_search for each user ID found in message text, then manually substitute. This is tedious and error-prone for consumers.

Suggested implementation

The server already has user data cached and resolves msg.User to names via getUserInfo(). The same lookup could be applied to <@USERID> patterns in msg.Text before returning results.

In pkg/handler/conversations.go, after this line:

Text: text.ProcessText(msgText),

Add a step to resolve <@USERID> patterns using the existing usersMap.