feat(sdk): add reviewed release notes and align Go timeouts

[moonyue-w]

Summary

GitHub Releases now publish reviewed notes from the matching CHANGELOG.md version instead of relying on an empty release description. Forward and Managed clients also bound the default wait for response headers to 10 minutes, matching the pinned Anthropic Go SDK baseline (v1.74.0).

• Add changelog validation/extraction, CI coverage, release-workflow integration, and contributor/PR guidance.
• Clone the default HTTP transport once per client and preserve custom HTTP clients and wrapped transports. The header timeout does not limit response-body reads or SSE duration; explicit request/context deadlines remain available.
• Record the behavior change under Unreleased and update README/API documentation.

Why

Each release needs a version-specific explanation that reviewers can approve. A server that accepts a connection but never sends response headers should also have a default waiting limit without cutting off an established event stream.

Verification

• go test ./...
• go test -tags live -run 'Cleanup.*Offline$' ./forward ./managed
• go vet ./..., Go formatting checks, and make build (including examples).
• Generated API reference checked for consistency, normalization, and internal links.
• Changelog validation and 19 release-notes automation tests.
• New conformance tests cover both clients, connection-pool reuse, custom-client precedence, wrapped transports, header timeout, and stream/body lifetime.

Account-backed integration tests were not run; the changed transport behavior is verified with local HTTP servers and offline tests. GitHub release publication itself was not executed locally.

Impact

The default response-header wait changes from unbounded to 10 minutes. Applications needing different limits can provide an HTTP client; whole-call and per-attempt deadlines remain independently configurable. API resource contracts and fixtures do not change. No credentials, live environment files, or test output are included.

Related SDK changes: Python #7, TypeScript #7.