Handle missing executable name in append-shared

[assisted-by-ai]

Summary

• avoid IndexError in append-shared when invoked without a mode by safely deriving the executable name
• fall back to the script name if the mode argument is missing

Testing

• python3 usr/libexec/helper-scripts/append-shared append /tmp/testappend foo

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Codex Task

[ArrayBolt3]

The idea here is good, but the implementation is not. We should be warning the user and exiting if someone tries to call append-shared directly. Will implement a different approach to this.

[ArrayBolt3]

Implemented: ArrayBolt3@3e071fd

[assisted-by-ai]

Added a safe executable-name helper in append-shared to prevent IndexError when the script is invoked without a mode argument, falling back to the script name as needed.

[ArrayBolt3]

Partially accepted and implemented as described above. This PR can be closed.