Remove dead exception handler in index_collections subprocess call

[Copilot]

Addresses feedback on #8 regarding unreachable exception handler in index_collections method.

Changes

• Removed dead except subprocess.CalledProcessError handler: The handler was unreachable because subprocess.run() was called without check=True. Error handling already exists via result.returncode checking.

• Fixed subprocess invocation: Changed from passing joined string to passing command as list directly. This was broken after shell=True was removed in commit 6c0942d, and the list form is more secure.

Before:

cmd_string = ' '.join(cmd)
result = subprocess.run(
    cmd_string,  # String without shell=True fails
    cwd=self.arclight_dir,
    ...
)

After:

result = subprocess.run(
    cmd,  # List form, no shell needed
    cwd=self.arclight_dir,
    ...
)

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