[v0.3] Add minibuffer foundation

[owainlewis]

Goal: Add a real minibuffer foundation for prompt-driven editor interactions.

Context: Cortex currently has a slash command line, but search, find-file, switch-buffer, and M-x need a reusable prompt surface.
This should be a small foundation, not a full completion system yet.

Proposed approach:

• Add minibuffer state to the app/editor state.
• Render the minibuffer through the existing bottom chrome/modeline area or a clearly separated bottom prompt row.
• Support prompt text, editable input, cursor movement within the prompt if practical, submit, cancel, and transient messages.
• Route prompt completion back to command handlers instead of creating separate behavior paths.
• Keep the slash command line behavior working, or explicitly fold it into the same prompt machinery if that keeps the code smaller.

Acceptance criteria:

• A command can open a minibuffer prompt with a label and current input.
• Printable keys edit the prompt without editing the buffer.
• Enter submits the prompt to a caller-provided command path.
• Escape cancels the prompt and restores normal editing.
• Status messages and dirty quit prompts remain visually clear.
• Existing slash commands still work.
• Existing editor cursor placement is not broken.

Verification:

• Run cargo test.
• Manually open a prompt, type, submit, cancel, and return to editing.
• Manually test slash commands, save, dirty quit, and terminal cleanup.

Out of scope:

• Fuzzy completion.
• File picker replacement.
• Search implementation.
• M-x command execution.
• Multi-buffer switching.