Translate ctx.bash.run() REJECTIONS into the SEARCH_* taxonomy. The seam
contract has run() reject for infrastructure failures (a pre-aborted
signal, an unusable/deleted session workdir, a missing shell); the bare
await let those escape as plain Errors, so the tool registry produced
isError results without the structured SearchError { name, code } the
package documents. A pre-aborted spec.signal now maps to SEARCH_ABORTED
and any other start failure to SEARCH_FAILED, original error chained as
cause. Covered by fake-executor tests for both branches plus real-executor
integration tests pinning the exact pre-aborted-signal and deleted-cwd
paths.
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