Add agent-shell-session-prompt-exclude-choices defcustom

[nhojb]

Adds agent-shell-session-prompt-exclude-choices defcustom to support filtering of the prompt choices when starting a new shell.

For example you can exclude the downloads and temp options:

(setq agent-shell-session-prompt-exclude-choices '(:downloads-shell :temp-shell))

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[xenodium]

Thanks for the PR.

Unfortunately, I may need to defer this PR for some time until I address some of the consolidation I mentioned in #605 (comment)

At present, I'm worried we may be adding too many customizations in this particular area. I'll wait and see if there's more demand for the filtering. Sorry.

[nhojb]

No problem, I understand.

I created the PR because I personally don't use the Downloads and temp choices, so wanted to reduce the noise in the new-session-prompt buffer. But it's not urgent at all.