More reason codes in Claude Code's internal diagnostics

Several of Claude Code's internal diagnostic messages now include a short reason code that says what went wrong. These are records kept for troubleshooting, not messages written for you. The additions include:

• session_not_found, for a session (one conversation with Claude Code) that could not be found
• auth_headers_wait_started, marking when Claude Code starts waiting for sign-in details
• no_project_files, a new reason string
• a shutdown marker added to the end of turns that were interrupted
• a credential count in the counts kept when files are filtered

The list of sign-in credential kinds is now also shared across more of the code.