Add shell completions for the dispatch CLI

[galligan]

Why

The PyPI package is installable now, but CLI completion is still part of making dispatch feel native in everyday shell use. Users should be able to enable completions without reading Typer internals or guessing which shell setup path applies.

Scope

Add or document supported shell completion setup for the dispatch command.

Acceptance criteria

• Document completion setup for the shells we intend to support initially.
• Verify the instructions work for an installed uv tool install outfitter-dispatch command, not only uv run dispatch from the repo.
• Include upgrade/uninstall implications if generated completion files need cleanup.
• Add tests or smoke coverage where practical, or clearly document why this remains manual.
• Update README/docs/skills if completion becomes a recommended setup step.

Notes

Keep this small. If Typer already gives us enough, prefer documenting the blessed path over building a custom completion installer.