Fix invisible picker rows on light-mode terminals

[asujithan]

Fix #159

Drop the fg:white overrides in both pickers' styles. Use noinherit alone for highlighted and selected, which strips prompt_toolkit's default per-row highlight without applying any colour of its own. The only colour signals left:

• Cursor row → cyan › pointer
• Selected rows → filled ● vs empty ○ circle
  Row text always renders in the terminal's default foreground, so the list is readable on both light and dark themes.

Changes

• src/ucode/ui.py — prompt_for_tools style
• src/ucode/mcp.py — _picker_style (shared across MCP pickers)

Test plan

• uv run ruff check .
• uv run pytest
• Manual: ucode configure and ucode configure mcp on a light terminal — all rows visible, cursor/selection signals work
• Manual: same flows on a dark terminal — no regression