New plugin hook events for tool checks, config changes, and sessions

What

• The plugin hook system (the mechanism plugin scripts use to observe or intervene in Claude Code's behavior) adds new event types: tool.check (a dry-run check of whether a tool call would be permitted, without actually running it), config.set and config.describe (observe or gate configuration changes), session.attach and session.detach (fired when a session's viewport/surface, or client, attaches or detaches, carrying details like surface, clientId, and detach reason), and plugin.register.
• tool.check handlers can respond with a decision of allow, ask, or deny (plus an optional reason/rule) for a given tool call.
• The old turn.complete handler was replaced with a new implementation.
• The SDK's own request-normalization layer also gained tool.check, config.set, and config.describe as pass-through operations, alongside existing ones like tool.describe and command.run.

Why These new event types let plugins and hooks do more than just intercept tool calls after the fact — they can pre-check permissions, watch config changes, and track session lifecycle, making more sophisticated plugin behavior possible.