Update Slack integration to use channelId instead of channelName

[spacecostsmoney]

Made a new configuration var in AWS Dev.

Turns out we can just pass channelId or name into the call this is already using. https://docs.slack.dev/reference/methods/chat.postMessage/

Will need an admin to add this in prod before merging!

[spacecostsmoney]

I'm not planning to fix the badge generation as that's not important for this :)

[Copilot]

Pull Request Overview

This PR updates the Slack integration to use channel IDs instead of channel names when posting messages. According to the Slack API documentation, the chat.postMessage method accepts either channel IDs or names, so this change maintains compatibility while moving to a more explicit identifier.

Key Changes:

• Renamed configuration parameter from SlackChannel to SlackChannelId
• Updated all function parameters from channelName to channelId throughout the codebase
• Updated documentation to reflect the use of channel IDs

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
template.yaml	Renamed CloudFormation parameter from SlackChannel to SlackChannelId with updated description
src/index.js	Updated environment variable reference from SlackChannel to SlackChannelId
src/slack-service.js	Renamed function parameters from channelName to channelId
src/slack.js	Renamed function parameter from channelName to channelId
test/index.test.js	Updated test variable and environment variable names to use slackChannelId
README.md	Updated documentation to reference channel ID instead of channel name

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