feat(harness): load Claude-format plugins whole only on the Claude harness

[xytian315]

Summary

1. Plugins whose manifest is .claude-plugin/plugin.json are now accepted by claude harness. The Claude harness keeps such a plugin intact and passes its directory to Claude Code with --plugin-dir, so its skills, commands, and subagents load the way Claude Code loads them.
2. Agent Plugins format packages are unchanged on every harness. When a plugin has both manifests, the root plugin.json is used.
3. The Codex harness and the ADK runtime cannot load a Claude-format plugin and now fail at startup with a message saying so.
4. For claude code, under the existing --bare and --strict-mcp-config flags, hooks and plugin MCP servers are not loaded, and subagents are visible but cannot be invoked because Agent is not in the bare tool allowlist. Those are unchanged by this PR and could be follow-up contributions if need.