## Summary
Adds read methods to `RunStore` (`findRun`, `findRunOrThrow`,
`findRuns`) and routes every Postgres read of `TaskRun` through them,
mirroring how writes already go through the store. Behavior-preserving:
each relocated read keeps its exact query, field selection, and database
client (writer, replica, or transaction). This lets `TaskRun` reads be
retargeted to a different backing store later without touching call
sites.
Stacked on #3981 (the write adapter); that PR is the base of this one.
## Scope
In scope: the run engine, webapp services, presenters, and route
loaders. Three reads that pulled `TaskRun` in through a parent model's
relation `include` (alert delivery, batch results, attempt-dependency
cancellation) are decomposed to fetch the run(s) through the store and
stitch them back, since a relation include would not follow `TaskRun` to
a new table.
Left reading the existing table (out of scope): the legacy MarQS paths,
the legacy trigger idempotency read, and one raw-SQL recovery script
(commented for revisiting at cutover).
## Notes
Reads default to the read replica; callers pass the writer or a
transaction client wherever the original read did, so writer-vs-replica
behavior is unchanged.
Chris Arderne