Add optional context budget/source/reference metadata to plugin hook contexts plus llm_output and sanitized model_call_* hook events.
Thread the existing resolved context-window info through Pi embedded runs, CLI harness runs, and Codex app-server hook emission so plugins can observe the effective budget after agent/model/config caps.
Document the metadata and cover the CLI, Pi, Codex app-server, and model-call paths with focused tests.
Fixes#64327.