fix(orchestrator): carry a failed batch's reason to the request error log

[behinddwalls]

Summary

When a batch failed, the request that could not land ended in error with an empty message: mergesignal and speculate recorded only the terminal batch state, and conclude terminated the request with an empty reason — so a "did not land: error" carried no diagnostic at all.

Record why the batch failed on the batch itself when it is marked failed: the merge result's reason on the merge path, and "no speculation path could pass" on the speculate path. Persist it through the batch store (new failure_reason column, new Batch.FailureReason field), and have conclude pass it to the request's terminal log instead of an empty string. The dead-letter path already threads its own reason and is unchanged.

The demo client's run summary also surfaces the reason, so a failed request reads id=error: <reason> instead of a bare id=error.

Test Plan

✅ mergesignal records the merge reason on the batch; conclude carries it to the request's terminal log (new test); the batch store round-trips failure_reason; the client summary includes the note. //submitqueue/... green, CI gates clean.

Stack

1. feat(demo): add BURST mode for a simultaneous enqueue of independent changes #615
2. @ fix(orchestrator): carry a failed batch's reason to the request error log #616