tri pr ready: a path-filtered workflow last run >15 master commits ago reads as NO BASELINE #5837

tri pr ready scores a failing check against the last 15 default-branch commits and the last few merged PRs. A workflow with a paths: filter that last ran on master more than 15 commits ago is absent from both, so the check reads NO BASELINE and the verdict is CANNOT TELL -- although master's own latest run of that workflow fails the same job the same way.

Measured 2026-10-04 on #5826 (a cli/tri/src/census.rs change):

• fpga-conformance failed: elaboration errors: 178 (baseline 176), NEW adapter, ternary_isa 6 -> 7, uart 0 -> 1.
• master's latest FPGA E2E Build run, at e7ed379 (21 commits back), fails fpga-conformance with the identical lines.
• tri pr ready 5826 -> CANNOT TELL -- 1 failure(s) have no baseline (exit 3).

Fix: for a failing check with no observation in the window, read the check run's own workflow, take that workflow's most recent completed run on the default branch that ran a job of the same name, and score the check by it -- printing the commit and date, so an old observation reads as old. Only for checks that would otherwise have no baseline; the 15-commit walk is unchanged.

Refs #5786