New functionHooks event: prompt.context

A new hook event, prompt.context, joins prompt.section, ui.render, and tool.describe in the SDK/plugin hooks event registry. A hook can now contribute named context blocks to a prompt via {blocks: [{name, text}]}, subject to a validator that caps the count at 32 blocks, requires unique names, and enforces a combined 32000-character budget on top of what the engine itself adds. The event is fully wired through the event list, the site/core hook dispatch tables, and new cache-invalidation hooks invalidatePromptContext and invalidateHookedPromptContext.