docs: clarify that managed agent outcome events are not supported in Go SDK

[Aaravanand00]

Summary :-
Adds a clarification note and a compilable Go example to address unsupported
managed agent outcome events in the Go SDK.

Context :-
The managed agents documentation includes a Go example using
user.define_outcome and related types that do not exist in
anthropic-sdk-go, causing confusion and compilation issues.

Changes :-

1. Add note in README.md clarifying that outcome events are not supported
2. Add a minimal working Go example (examples/session-basic/main.go)
3. That uses only supported SDK APIs

Result :-

1. Prevents confusion around non-existent types
2. Provides a working reference for Go users
3. Ensures example code compiles successfully

Fixes #320