[Advanced] Go client for contextio-sdk (feature parity with the TS SDK)

[Eras256]

Context
Same gap as the Python client issue, for Go: contextio-sdk (packages/sdk/src/client.ts) only exists in TypeScript today.

Scope
A Go module mirroring the real, current surface of ContextioClient:

• Challenge(address string) (WalletChallenge, error)
• Verify(...) (WalletSession, error) (same input shape as the TS verify())
• WithSession(session) — client bound to an existing session
• Treasury(), Obligations(), Decisions(), Propose(), Legal()
• WellKnownLegalContext(domain)

SEP-53 signing is out of scope — accept a caller-supplied signing function; do not reimplement Stellar keypair signing.

Acceptance criteria

• A real Go module (go.mod) under this repo, e.g. packages/sdk-go/.
• A runnable example (cmd/example or similar) completes challenge → verify → treasury() → obligations() against the live public testnet demo API (https://contextio-api.fly.dev).
• Unit tests cover request/response shapes with mocked HTTP.
• README documents install + the runnable example.

References
packages/sdk/src/client.ts, packages/sdk/src/types.ts for the exact shapes to mirror.