Add Codex MCP integration

[arsis-dev]

Summary

• Add Codex to the MCP integration client list.
• Wire Codex installation through codex mcp add tabularis -- <tabularis> --mcp.
• Show the Codex-specific manual command in the MCP setup UI instead of reusing the Claude Code command.
• Add regression coverage for the Codex client entry and command generation.

Context

The MCP integration page already supports Claude Desktop, Claude Code, Cursor, Windsurf, and Antigravity. Codex also supports MCP through codex mcp add, but Tabularis did not expose it as an install target.

I searched existing open issues and PRs for Codex MCP and OpenAI Codex mcp server integration; I did not find a duplicate.

GitNexus

• npm exec --yes gitnexus -- detect-changes -> 3 files, 17 symbols, risk level medium, 4 affected UI flows through McpPage.
• npm exec --yes gitnexus -- impact install_mcp_config --direction upstream -> risk low, partial result due GitNexus read-only pool warning.
• npm exec --yes gitnexus -- impact get_mcp_status --direction upstream -> risk low, partial result due GitNexus read-only pool warning.
• npm exec --yes gitnexus -- impact McpPage --direction upstream -> risk low, impact reaches App.

Validation

• cargo test mcp::install -> 3 passed
• pnpm run typecheck
• git diff --check
• pnpm tauri dev starts successfully

[debba]

Thanks for this.
I will merge it now.