Supports effort passthrough overview

The agent forwards effort-level requests to the bridged harness rather than handling effort tiers natively in its own core. Used by orchestration layers (e.g. a5c) that dispatch to multiple harnesses and rely on each harness's native effort taxonomy. Distinct from `capability:supports-thinking` (which is about thinking-block surfacing) and from native `supports_effort_level` edges (which assert per-tier support).