Don't resubscribe a project member when they leave a channel

Right now, team automatically adds project members to certain Zulip channels such as all/private. When a project members unsubscribes from these channels, team resubscribes them.
This shouldn't happen.

When team noticed a difference between subscribers to a channel and Rust Project members, team should distinguish these two cases:

• a new project member joins the Rust project, and so they need to be added to these channels.
• an existing member of the project unsubscribed from one of these channels.

Kobzol's proposal:

the solution for that is make the subscription edge-triggered, rather than level-triggered; subscribe once the person joins the team, unsubscribe once they leave. and if they unsubscribe themselves, just give them the possibility to resubcribe, but don't force it

Start by tracing the team logic that compares Zulip channel subscribers with Rust project members. Verify how it handles a new member versus an existing member who unsubscribes, then confirm that only membership changes trigger automatic subscription while voluntary unsubscribes remain possible.