error-app-exits hint: the test runs without network on purpose, and so must the application

[probonopd]

Replaces #10018 (withdrawn: the catalog never runs applications with network access, not even to diagnose). error-app-exits is the largest failure class (1,931 open PRs); its hint now says what the rule is and what to do:

The application quit or crashed right after starting; the error below usually shows why. Note that the test runs the application without network access, on purpose, and that this is a requirement for the catalog: an AppImage must start without network (users are offline too). If the application needs a server, a login or an update check to start, please make it show its window (or at least a message) first and connect afterwards, and handle a failed connection instead of exiting.

The README's "After you submit" section gets the same rule in one sentence. No change to the test itself.

Checked: bash -n, shellcheck, the hint appears in diagnose.sh --list (the publish workflow validates hints against that list from master, so it shows up in test results after merging).

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https://claude.ai/code/session_01DZvaN9tm5taf5avUvVtmyq

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