Add build_admission_context section to nightly-pr-digest.sh that renders
GH evidence status, main CI baseline, open PR blocker count, blocker PR
table, and admission verdict (dry-run/admitted/stopped). The JSON format
includes the full admission_context object from digest.json.
BATS tests verify the PR body markdown contains the admission table and
the JSON format exposes admission fields.
Closes soc-y8b.11.5.
Reads digest.json, cycle-history.jsonl, fitness baselines, bd tracker,
and git log to produce a markdown PR body covering baseline/final goals,
cycle history, runtime-artifact flips, open PR overlap, tracker
degradation, transient flakes, auto-reverts, and tag-push status.
Wired into nightly-evolution.sh as an automatic post-run step.