Plugin hook engine adds turn.start, a managed-settings notice and skill frontmatter

What

Hooks are small pieces of code a plugin runs at set points while Claude Code works. The engine that runs them changed in several ways.

• turn.start now counts, together with turn.step and turn.complete, when Claude Code decides whether any hooks are active. A try/catch that called an error handler was removed in the same area.
• A new status explains that, where an organization manages Claude Code, a plugin's hooks on certain events do not run: "these events pass over the plugins you install". Only the organization's own plugins run hooks on those events.
• A new tool.register hook event is added.
• A plugin with a module path (modulePath) now counts as having hooks in the reports sent when a plugin loads or is enabled. Before, only hooksConfig counted.
• The engine gains restoreArgument for prompt.submit and skill.prompt.
• A check on frontmatter data, the settings block at the top of a skill file, rejects prototype keys, proxies, cycles and accessors.
• skill.prompt now also receives the skill's frontmatter as written in the file, a frontmatterProblem field, and the skill's source, plugin name and path. Before, it received only { skill, text }. If the frontmatter cannot be copied, it gets {} and a warning is logged.
• Separately, several functions that write session files return early in what looks like a diskless session mode. One throws "A diskless session writes no side file to disk".

Why

Plugin authors can react to the start of a turn and read or rewrite a skill's frontmatter. People on managed installs get a reason when their own plugins' hooks do not run.