fix(cli): dynamic shell completion with API-driven resource names

[DerDennisOP]

Summary

• Switch the CLI to clap_complete's dynamic completion engine so gradient <TAB> actually works. The old static script registered against the capitalised Gradient app name instead of the gradient binary, which silently disabled completion entirely.
• Add server-backed completion of existing resource names: organizations, projects, workers, and caches (selectors for select/show/edit/delete/remove, plus organization cache add). Lookups honour the partial prefix and the currently selected organization.
• Completers run before the tokio runtime starts, each spinning up a short-lived current-thread runtime with a 2s timeout. They fail silently (return no candidates) when offline, logged out, or on any non-2xx/network error, so the shell never errors.
• gradient completion <shell> re-invokes the binary with COMPLETE=<shell> to emit the dynamic registration stub, keeping the Nix entry point unchanged.

Test plan

• cli/tests/completion.rs — gradient completion {bash,zsh} emits a script registered against the lowercase gradient binary (-F _clap_complete_gradient gradient) and never references Gradient.
• wiremock-backed unit tests in cli/src/commands/completion.rs — each completer returns resource names, respects the prefix, scopes to the selected org, and yields no candidates on a non-2xx response.
• cargo clippy --manifest-path cli/Cargo.toml --all-targets passes clean.