New coordinator_context and context_sections system-prompt injectors

What

• Two new context-injection types, coordinator_context and context_sections, are added to the existing set of system-prompt injectors (environment, model, session_context, etc). coordinator_context injects the list of worker tools available to a coordinator in multi-agent ("coordinator/worker") setups, and context_sections injects arbitrary named text sections (each with a name and text).
• These same two injectors are also available as message attachments when building a conversation, with coordinator_context only injected when a new gate check passes.
• The session-refresh logic used when resuming a session now builds a flat list of {name, text} sections (merged from user and system context) instead of separate fields, and always marks refreshed context as bare: true on the error path instead of silently returning stale context.
• A gate that used to suppress the environment/model/output_style/session_context/instructions/date/language injectors entirely has been removed, so these now always run.

Why This lets Claude Code pass structured, named context (like a coordinator's list of worker tools) into the system prompt in a more organized way, and makes session resumption more reliable by always refreshing context sections instead of occasionally reusing stale data.