feat: Allow xb-cspell to check only staged files

[penyaskito]

Fixes #74.

Adds an optional staged argument to xb-cspell so it can lint only the files staged for commit — useful for pre-commit hooks.

Behavior

• ddev xb-cspell — unchanged; lints the whole module via npm run lint:cspell.
• ddev xb-cspell staged — runs cspell against git diff --cached files only (--diff-filter=ACMR, so deletions are excluded). Exits cleanly with a message when nothing is staged.
• Any other argument prints an error and exits non-zero.

Also adds staged to AutocompleteTerms.

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