feat: add coverage summary to GitHub Actions job summary

Write a markdown coverage table to $GITHUB_STEP_SUMMARY for both unit
and E2E test jobs. Uses a small bash script that parses json-summary
output from the coverage reporters.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/coverage-summary.sh

@@ -0,0 +1,37 @@
+#!/usr/bin/env bash
+# Writes a coverage summary table to $GITHUB_STEP_SUMMARY (or stdout if not in CI).
+# Usage: scripts/coverage-summary.sh <title> <json-summary-path>
+set -euo pipefail
+
+TITLE="${1:?Usage: coverage-summary.sh <title> <json-summary-path>}"
+JSON="${2:?Usage: coverage-summary.sh <title> <json-summary-path>}"
+
+if [ ! -f "$JSON" ]; then
+  echo "Coverage summary not found: $JSON" >&2
+  exit 0
+fi
+
+# Resolve to absolute path for require()
+ABS_JSON="$(cd "$(dirname "$JSON")" && pwd)/$(basename "$JSON")"
+
+# Extract totals with node (available in CI, no jq dependency)
+read -r stmts branches funcs lines < <(node -e "
+  const s = require('$ABS_JSON').total;
+  console.log(s.statements.pct, s.branches.pct, s.functions.pct, s.lines.pct);
+")
+
+TABLE="## $TITLE
+
+| Metric | Coverage |
+|--------|----------|
+| Statements | ${stmts}% |
+| Branches | ${branches}% |
+| Functions | ${funcs}% |
+| Lines | ${lines}% |
+"
+
+if [ -n "${GITHUB_STEP_SUMMARY:-}" ]; then
+  echo "$TABLE" >> "$GITHUB_STEP_SUMMARY"
+else
+  echo "$TABLE"
+fi