feat: adopt Agent Plugin spec and generate harness manifests

[wangauone]

Summary

Adopts the vendor-neutral Agent Plugin spec as the single source of truth for this extension and generates the per-harness manifests from it, so one plugin installs across Gemini CLI, Claude Code, Codex, and Antigravity.

Stacked on chore/npx-toolbox (base branch). Review/merge that first.

What changed

• Spec source: root plugin.json (no $schema, so Codex routes to the generated .codex-plugin/) and mcp.json (MCP server via npx @toolbox-sdk/server).
• Generated harness manifests:
  • Gemini CLI , gemini-extension.json (settings prompt to .env)
  • Claude Code , .claude-plugin/plugin.json (config via userConfig / --config)
  • Codex , .codex-plugin/{plugin.json,.mcp.json} (env forwarded via env_vars)
  • Antigravity , mcp_config.json (reads ambient shell env)
• Marketplace: .claude-plugin/marketplace.json , used by both Claude Code and Codex (Codex reads it via its discovery fallback), so no drift.
• README: installation and configuration documented for all four harnesses.

Verification

Loaded the generated plugin on all four harnesses (plugin + skills load, MCP server starts, config/env resolves). Empirically confirmed each harness's config delivery: Gemini .env, Claude userConfig (ambient shell env is shadowed to empty for wired keys, so --config is required), Codex/AGY ambient shell env.