Task: Prove the pipeline with both ends substituted

Run a definition whose configuration substitutes both pre-processing and post-processing, and show that the middle of the pipeline executes for real: the scheduler fires, the workflow builds the substituted chain, the grid runs the engine, the output reaches storage, and the run records that it was substituted.

• The instance reaches a terminal state through the state machine, and records which phases were substituted.
• The engine runs and its output object exists in storage, on a database recreated from scratch.
• The run is kept as a repeatable check, and it proves nothing about the phases it replaced.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Every number this task claims about a tree, with the command that produced it and the commit it was measured at. A number without both is not a measurement, because a clean-up record describes a tree and trunk moves under a branch: a consumer count, a gate's component count and a suite's size all change with no code changing, and the reader of a bare number cannot tell which tree it came from. Where the raw output matters, put it in an evidence/ file beside the task and name that file in the Command column.

Manual QA scenarios (scaffolded via compass add test_scenario) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.