Adds a new HTTP GET handler at the ingestion pod that walks
brain/sessions/*.jsonl, filters by skill name and timestamp window
(default 7d, accepts Nh and Nd), normalizes legacy status vocabulary
(ok->pass, error->fail, skipped->skip), and returns aggregated counts
plus pass_rate.
Pass rate is null when pass+fail == 0, distinguishing 'no data' from
'always passes'. Plan 6 routing pod will check for null before
making decisions.
Route registration in cmd/server/main.go lands in a follow-up commit.
