fix(vscode): pass workspace directory in MCP requests so browser tools register in the correct project state

MCP HTTP methods (addMcpServer, disconnectMcpServer, getMcpStatus,
connectMcpServer) were not sending the x-opencode-directory header.
The server fell back to process.cwd(), creating the Playwright MCP
client in a different Instance.state() bucket than the one used when
the agent processes messages. The browser tools appeared registered
but were invisible to the agent.

Fixes #292