fix gemini mcp registration to write to correct settings.json

[asujithan]

Summary

Before:
ucode configure mcp would silently register Gemini MCP servers into a
file the launcher never reads, so /mcp inside ucode gemini always
showed "No MCP servers configured." even after a successful save.

add_gemini_mcp_server / remove_gemini_mcp_server ran gemini mcp add/remove without setting GEMINI_CLI_HOME, so entries went to
$HOME/.gemini/settings.json. But agents/gemini.py launches Gemini
with GEMINI_CLI_HOME=~/.ucode/.gemini-home, which makes the session
read from a different settings.json. The two never intersected.

Fix: pin GEMINI_CLI_HOME to the same gemini.GEMINI_HOME_DIR for
both subprocess calls so registrations write to the file the launcher
reads.

Test plan

• uv run ruff check src/ucode/mcp.py tests/test_mcp.py
• uv run pytest tests/test_mcp.py (39/39)
• ucode configure mcp → select an entry → ucode gemini →
  /mcp lists the registered servers and tools/list works.
• ~/.ucode/.gemini-home/.gemini/settings.json contains the new
  mcpServers entries; ~/.gemini/settings.json is no longer
  written by ucode.