feat: add tab completion for commands in interactive shell

[skyc1e]

Adds tab completion for commands and subcommands in the interactive shell (issue #34).

The completer pulls the command tree directly from clap's CommandFactory, so it stays in sync automatically when new commands are added. No hardcoded lists.

Pressing Tab on an empty prompt shows all available commands. Typing a command + space + Tab shows its subcommands (e.g. clob → book, price, spread, ...). shell and setup are excluded since they're blocked in the REPL.

No new dependencies — just uses rustyline::completion and clap::CommandFactory that are already in the tree.

Closes #34

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Note

Low Risk
Low risk: changes are isolated to the interactive shell input layer and only affect readline behavior (completion/editor initialization), not command execution or data handling.

Overview
Adds tab-completion to the interactive shell by introducing a rustyline helper (ShellCompleter) that derives available commands/subcommands from clap’s Cli::command() (excluding shell and setup).

Updates run_shell() to use rustyline::Editor with the new helper and prints a completion hint, enabling Tab completion for top-level commands and their immediate subcommands.

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