Apply ExcludeRefPrefixes to probe output

probe accepted --exclude-ref-prefix and ProbeRequest.ExcludeRefPrefixes since the flag landed, but newProbeResult serialized every ref from s.sourceRefMap verbatim — so probe --all-refs --exclude-ref-prefix refs/pull/ still listed refs/pull/* in the JSON output, breaking probe as a way to preview the same effective ref set that sync/fetch would act on.

Filter in newProbeResult so the contract holds across all command paths. Regression test goes through the CLI: seeds refs/pull/1/head and refs/notes/commits on source, runs probe --all-refs --exclude-ref-prefix refs/pull/ --json, asserts pull is absent and notes is present.