feat: add reactions_list tool to list items user has reacted to

[dankorotin]

Summary

• Add new reactions_list MCP tool that wraps Slack's reactions.list API endpoint
• Lists all items (messages, files, file comments) that the authenticated user has reacted to
• Can be used as a workaround for accessing saved/bookmarked items since Slack's "Save for Later" API is not publicly available

Changes

• pkg/handler/conversations.go: Add ReactionsListHandler, convertReactedItems, and parseParamsToolReactionsList functions
• pkg/provider/api.go: Add ListReactionsContext to SlackAPI interface and MCPSlackClient implementation
• pkg/server/server.go: Register the new reactions_list tool with parameters

Parameters

Parameter	Type	Default	Description
count	number	100	Number of items to return per page (max 1000)
page	number	1	Page number for pagination
full	boolean	true	If true, return full message/file details

Response Format

Returns CSV with columns:

• type - item type (message, file, file_comment)
• channel - channel ID
• timestamp - message timestamp
• emoji - reaction emoji name
• text - message text or file name (truncated)
• userID - author's user ID
• userName - author's username
• time - ISO 8601 formatted time
• cursor - next page number (if more results available)

Test plan

• Build the project: go build ./cmd/slack-mcp-server
• Run with valid Slack credentials
• Call reactions_list tool via MCP client
• Verify pagination works with page parameter
• Verify results include messages, files the user has reacted to

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