Expose context-size summaries for subagent runs

Problem\n\nThe main run exposes context-window information, but delegated subagent activity does not provide comparable context-size summaries. Operators cannot tell how much context a subagent is using or how close it is to its resolved window.\n\n## Desired behavior\n\nReport concise per-subagent context summaries through the existing delegation observability surface, so clients can display context usage/window information while a subagent runs and/or at its terminal outcome.\n\n## Scope to determine\n\n- Define the summary fields, including estimated current request tokens, resolved context-window size, and whether estimates are live or terminal-only.\n- Thread the data through subagent lifecycle events without leaking child conversation content into the parent.\n- Cover foreground, background, resumed, direct-write, and model-override subagents; specify whether Parallel branches and team members should share the same projection.\n- Keep the context-window source consistent with the existing per-model, live-resolved context-window closure.\n\n## Acceptance idea\n\nOffline child-run coverage proves a subagent lifecycle projection contains the expected context summary for its resolved model, and client rendering shows it without altering the parent conversation.