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>

.mcp.json

@@ -0,0 +1,8 @@
+{
+  "mcpServers": {
+    "skill-seekers": {
+      "command": "python",
+      "args": ["-m", "skill_seekers.mcp.server_fastmcp"]
+    }
+  }
+}