docs: add doc.go for pkg.go.dev documentation

[ojongerius]

Problem

pkg.go.dev shows "There is no documentation for this package" for our Go modules:

• https://pkg.go.dev/github.com/agent-receipts/ar/mcp-proxy
• https://pkg.go.dev/github.com/agent-receipts/ar/sdk/go

Solution

Add doc.go files with package-level comments to:

• mcp-proxy/doc.go
• sdk/go/doc.go (or per-package: receipt/doc.go, store/doc.go, taxonomy/doc.go)

Documentation will appear on pkg.go.dev after the next tagged release.