add public channel managers endpoint

[deployor]

This allows any bot or person to look up channel managers without having to use a self-bot, even in private channels.

This means that bots such as at-channel, bChannel, etc. can rely on channel managers rather than the creators of the channel for private channels. This makes permission checks much easier!

[Copilot]

Pull request overview

Adds support for querying Slack “channel manager” role assignments via the existing proxy + pipeline service, enabling downstream bots/services to rely on channel managers (including for private channels) rather than channel creators.

Changes:

• Extend the proxy allowlist to permit admin.roles.listAssignments calls.
• Add the required Slack app OAuth scope (admin.roles:read) to the manifest.
• Introduce a new GET /channel-managers endpoint in the pipeline service that pages through role assignments and returns manager user IDs for a channel.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
proxy/app.py	Allows the proxy to forward admin.roles.listAssignments using the admin credential.
pipeline/slack.manifest.yaml	Adds admin.roles:read so the Slack app can call role assignment APIs.
pipeline/service/app.py	Adds a public /channel-managers HTTP endpoint backed by proxy calls to Slack admin role assignment APIs.

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