feat(slack): replace email references with Slack @mentions

[ptone]

Summary

• Port resolveOutboundMentions() from Telegram broker to the Slack broker
• Uses the same regex, URL/mailto guards, and reverse-order processing
• Replaces user:email and standalone email references with Slack <@user_id> mentions via GetUserMappingByEmail() store lookup
• Wired into Publish() before message formatting

Test plan

• Port all test cases from Telegram: user:email, standalone email, unknown user, URL email, mailto, multiple emails, edge positions
• All 13 test cases pass
• Full Slack package test suite passes

Partial #514

[gemini-code-assist]

Code Review

This pull request introduces outbound mention resolution in the Slack broker, automatically replacing Scion user emails with Slack @mentions in outgoing messages. A review comment points out a potential data race issue caused by mutating the shared msg pointer in-place, and suggests performing a shallow copy of the message struct to prevent unexpected side effects.

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[gemini-code-assist]

Mutating msg.Msg in-place modifies the shared msg pointer. Since Publish can be called concurrently across different brokers, or sequentially where other components rely on the original message content (e.g., for dedup key generation or logging), mutating the message in-place can lead to data races and unexpected side effects.

Additionally, msg is already checked for nil earlier in this function (line 363), making the msg != nil check redundant.

Instead, perform a shallow copy of the message struct and update the local pointer.

Suggested change

	if msg != nil && store != nil {
	msg.Msg = resolveOutboundMentions(ctx, store, msg.Msg)
	}
	if store != nil {
	msgCopy := *msg
	msgCopy.Msg = resolveOutboundMentions(ctx, store, msg.Msg)
	msg = &msgCopy
	}

[ptone]

Superseded by squash-merged upstream PR. Closing fork PR.