feat: CD command

[xxori]

Added aucpl cd, allowing changing directory to problem directories, or the root directory if no argument is passed. However, by default this will just print the directory. You must run eval $(aucpl shellinit) first (or add it to your zshrc or bashrc) to enable the actual directory-changing behaviour. This is because the CLI executable can't change the working directory of the parent.

[Copilot]

Pull request overview

Adds shell integration to support an aucpl cd subcommand that prints a directory (intended to be used by a shell wrapper to actually cd), and documents the workflow in the README.

Changes:

• Add aucpl cd subcommand and wire it into the CLI command registry/dispatch.
• Add aucpl shellinit subcommand that prints a shell function wrapper enabling aucpl cd to change the current shell directory.
• Document the new cd usage and shell hook in README.md.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
crates/cli/src/main.rs	Adds dispatch cases for cd and shellinit subcommands.
crates/cli/src/cli/mod.rs	Registers cd and shellinit in the builtin subcommand list.
crates/cli/src/cli/cd.rs	Implements aucpl cd directory resolution and printing.
crates/cli/src/cli/shellinit.rs	Implements aucpl shellinit printing a shell wrapper function.
README.md	Documents aucpl cd and how to enable shell integration via shellinit.

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