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.

[AarushiShah-db]

thank you!!