feat: add ManagedBy build flag to disable auto-updates - #117

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Description

Add the ManagedBy build flag to let third-party package managers (e.g., Nix) inform the CLI of their presence.
When a package manager is used, the CLI defaults disable_update_check = true.

How to Test

1. Build the CLI with ManagedBy set (e.g., -ldflags "-X github.com/fosrl/cli/internal/config.ManagedBy=nix").
2. Run pangolin config show and see disable_update_check defaults to true.