Update documentation for SDK 2.0 / 2026-07-28 spec #489

Update documentation for SDK 2.0 / 2026-07-28 spec

Part of #481.

Tasks

• doc/sdk-integration.md — SDK 2.0 version, capability changes.
• doc/machine-readable-contract.md — ServerFeatureSupport changes (AsyncTasks, McpLogging, Sampling).
• doc/capability-not-available.md — error-code/capability updates from #SUB6.
• doc/sdk-v1.1-compatibility.md — note 2.0 migration and supersession.
• README.md — update any SDK version references and feature claims.
• CHANGELOG.md — record the SDK 2.0 upgrade.

Acceptance

• Docs reflect the SDK 2.0 surface and the project's negotiated-version behavior.