[Feature]: Fuzzy/prefix filtering in the slash-command menu

[shauryagangrade]

Problem

The interactive slash menu (/ then arrow keys) lists 11 commands with no
way to filter. Users who remember part of a command (e.g. "ollama") have to
scroll and read every entry.

Proposed approach

• Add as-you-type filtering to _show_slash_menu: typing narrows choices
  (prefix and/or fuzzy match), Enter runs the top match.
• prompt_toolkit/questionary support custom instructions; keep the Esc
  cancel and arrow-key behavior.

Where to look

• gcode/ui.py:96-117 (_show_slash_menu), :23-36 (_SLASH_COMMANDS).

Acceptance criteria

• Typing "oll" filters to /ollama and related entries.
• Arrow keys and Esc still work; empty selection still returns "".

Difficulty

Low — good first issue.

[aryansk]

Working on this — enabling questionary's built-in search filter so typing narrows the slash menu (e.g. "oll" → /ollama) while arrow keys, Enter, and Esc keep working.