[Docs] Add doc.go files for pkg.go.dev documentation

mcp-proxy/doc.go

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+// Package mcpproxy provides an MCP (Model Context Protocol) proxy server
+// that handles authentication, routing, and request transformation for
+// MCP-compatible agent tools.
+package mcpproxy

sdk/go/receipt/doc.go

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+// Package receipt provides types and functions for creating, validating,
+// and verifying agent receipts - cryptographic attestations of agent actions.
+package receipt

sdk/go/store/doc.go

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+// Package store provides storage backends for agent receipts,
+// including in-memory and persistent implementations.
+package store