feat: add Codex CLI plugin manifest (#350)

Add Codex plugin support for discovery via codex-plugin-scanner:
- .codex-plugin/plugin.json — plugin manifest
- .mcp.json — MCP server config (starts server_fastmcp)
- skills/skill-seekers/SKILL.md — bundled skill for Codex
- .gitignore — allow root .mcp.json to be tracked

Co-authored-by: internet-dot <28622406+internet-dot@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -66,6 +66,7 @@ htmlcov/
 skill-seekers-configs/
 .claude/skills
 .mcp.json
+!/.mcp.json
 !distribution/claude-plugin/.mcp.json
 settings.json
 USER_GUIDE.md