[v0.3] Add command registry and M-x

[owainlewis]

Goal: Add a small command registry and M-x so Cortex has one discoverable command path instead of growing ad hoc command entry points.

Context: The PRD says every action should resolve to a named command and that the command registry should power M-x and key remapping later.
The current slash command surface is useful, but it is not the long-term Emacs-shaped command surface.
This should land after the minibuffer foundation in #28.

Proposed approach:

• Introduce a static command registry with stable command names, descriptions, and dispatch targets.
• Route existing keybound commands and slash commands through the same registry where this keeps the code smaller.
• Add M-x on top of the minibuffer.
• Show command names in a plain completion or filtered list only if that is cheap with the current minibuffer.
• Preserve the existing slash commands until they can be removed or folded in deliberately.

Acceptance criteria:

• Every existing editor action has one stable command name.
• M-x opens a minibuffer prompt for commands.
• Submitting a known command dispatches the same behavior as its keybinding or slash command.
• Unknown command input reports a clear error without editing the buffer.
• Existing keybindings, slash commands, save, quit, picker, and search behavior still work.
• The command registry is small, static, and easy to inspect in tests.

Verification:

• Run cargo test.
• Manually run at least save, quit cancel, open file, undo, and search through M-x where available.
• Manually confirm normal editing resumes after cancel and after unknown command input.
• Manually confirm the shell is usable after exit.

Out of scope:

• User remapping.
• Fuzzy command completion unless the minibuffer already makes it cheap.
• Scripting.
• Plugin hooks.
• Command history.