Add shell completions (bash/zsh/fish)

[sanmaxdev]

Tab-completion makes the CLI feel polished and lowers the barrier to exploring commands and flags.

What to do

• Add completion for the sentineldeck CLI (subcommands and flags), e.g. via argcomplete or generated completion scripts.
• Document how to enable it in the README.

Acceptance

• Working completion for at least bash; zsh/fish welcome.
• README documents enabling it; tests where practical.

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