CLI: report clear error when inspect finds no feed and add spec

[id774]

Motivation

• The inspect subcommand currently raises an internal exception when feed autodiscovery returns no feeds, which is not friendly for CLI users.

Description

• Make inspect_url raise Automatic::Error with a clear message when Feedbag.find returns an empty list by adding raise Automatic::Error, "no feed found at #{url}" if feeds.empty?.
• Add a spec in spec/lib/automatic/cli_spec.rb that stubs Feedbag.find to return [] and asserts the command returns EXIT_FAILURE, emits the expected error text, and produces no stdout.
• Update doc/VERSIONS to document the new behavior and bump file header update timestamps in the modified source and spec files.

Testing

• Ran rspec spec/lib/automatic/cli_spec.rb and the suite containing the new inspect test passed successfully.

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Codex Task