Fixes to container detection, CPU usage reading and update channel handling

Several small changes:

• Docker detection now reports "not in Docker" without checking for the /.dockerenv file when a certain internal condition holds.
• Reading CPU usage from Linux cgroups (a Linux feature that groups programs to track their resource use) now returns the path as listed, including the root path /, and no longer reads a value that was never set.
• Tool names in a transcript line now go through a name formatting step.
• How Claude Code works out the Homebrew update channel was rewritten.
• A comparison against a sent value is now protected so that an error counts as a match.