MCP client: bridge external MCP servers as built-in tools (closes #38)

[lezama]

Closes #38

Summary

Adds a wp-admin → openclaWP → MCP Clients page that lets admins register external MCP servers and have their advertised tools surface in the local ability registry under mcp/<server>/<tool>, so agents can call them just like any other ability.

What ships

• OpenclaWP_Mcp_Client_Store — CPT-backed store for server configs (transport, command/args/env or url/headers, per-tool allowlist + disabled list, cached tool metadata, last-error).
• OpenclaWP_Mcp_Client_Transport — thin MCP 2025-06-18 JSON-RPC client. Implements initialize + tools/list + tools/call over stdio (proc_open) and over Streamable HTTP (wp_remote_post).
• OpenclaWP_Mcp_Client_Bridge — registers each enabled server's non-disabled, allowlisted tools as WP abilities under mcp/<slug>/… and proxies execution back through the transport. Errors surface as WP_Errors so the loop's tool-call mediator can degrade gracefully ("the X tool is unavailable…").
• OpenclaWP_Mcp_Clients_Admin — list / add / edit / re-test / per-tool enable-disable / delete. Three one-click recipes:
  • Fetch — npx -y @modelcontextprotocol/server-fetch, works out of the box.
  • Context7 — npx -y @upstash/context7-mcp, needs CONTEXT7_API_KEY.
  • GitHub MCP — npx -y @modelcontextprotocol/server-github, needs GITHUB_PERSONAL_ACCESS_TOKEN.

No new PHP or JS dependencies. The transport uses PHP's built-in proc_open and WordPress's wp_remote_post.

Test plan

• vendor/bin/phpunit --testsuite unit — 41 tests, 88 assertions, all green (9 new tests under tests/unit/McpClientBridgeTest.php).
• php tests/smoke.php — exits 0 (full smoke needs a real WP via studio wp eval-file).
• npm run lint — clean.
• npm run build — webpack compiled successfully.
• Manual on a Studio site: install the Fetch recipe, click Re-test, see fetch tool advertised, attach mcp/fetch/fetch to an agent's default_config['tools'], ask the agent "fetch https://example.com" — agent receives the markdown.
• Graceful degradation: disable Fetch (e.g. wrong command), confirm the agent reports the tool is unavailable instead of hanging.
• Per-tool disable: from the edit page, disable an individual tool, confirm it disappears from the agent's tool surface on next request.

What's deferred

• HTTP transport is wired but unproven against a real Streamable HTTP MCP server.
• Catalog-mode integration with Multi-turn agentic loop with tool calling #1 (tool-discovery meta-tools) — bridged abilities all live under the openclawp-mcp-clients category so a future catalog layer can filter them as a group.
• OAuth flow for MCP servers that require it. The HTTP path supports static bearer headers via the config UI today.

Hard rules

• No edits to vendor/automattic/agents-api/.
• No .mcp.json at the repo root — the admin page is the canonical surface.
• composer.json unchanged. package.json unchanged (lockfile has only peer-flag reclassifications from npm install).