feat(shell-hook): validate --shell against the shells it accepts

[hunger]

--shell was a plain Option<ShellEnum> with the accepted names written out by hand in the doc-string, so the list went stale. rattler_shell gained the bash flavors sh, dash, ksh and busybox, plus brush, nu and powershell_ise, while --help and the CLI reference still named only the original seven. -s sh worked and nothing in pixi said so. A typo got a bare parse error with no alternatives, and completion had no candidates to offer.

Listing the names ShellEnum's FromStr accepts in a PossibleValuesParser lets clap reject an unknown shell with the alternatives, show the set in --help, and offer it in every generated completion script. The doc-string drops the hand-maintained list and states the default instead.

The list copies names rattler_shell cannot enumerate, so two tests hold it in step: every listed name must still parse, and an exhaustive match over ShellEnum plus a variant count catches a shell no listed name reaches. Neither notices a new alias for an existing variant, and the flavors behind ShellEnum::Bash cannot be guarded the same way because BashFlavor is #[non_exhaustive].

Refs #5495

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Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
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