Add Codex platform support

[clcc2019]

code-review-graph install: error: argument --platform: invalid choice: 'codex' (choose from claude, claude-code, cursor, windsurf, zed, continue, opencode, antigravity, all)

[tirth8205]

Skipping auto-merge: after updating against current main, CI shows this conflicts with existing Codex support that already landed via #177.

Specific failures:

• code_review_graph/skills.py:126 — F601 duplicate "codex" key in PLATFORMS dict (both this branch and main add it)
• tests/test_skills.py::TestInstallPlatformConfigs::test_install_codex_preserves_existing_toml — KeyError: 'code-review-graph'
• tests/test_skills.py::TestInstallPlatformConfigs::test_install_all_detected — AssertionError: assert 'Codex' in ['Claude Code', 'OpenCode']

Since #177 already delivers Codex MCP install support, this PR is effectively duplicate work. Could you rebase and either (a) drop the overlap and keep only the extras here, or (b) close this if #177 covers everything you needed?