ci(coverage): add auto-updated "core coverage" badge

Surface the metric ADR 0004 says matters — the critical security/logic
core, currently 95% — as a README badge, distinct from the
informational global `coverage` badge.

- scripts/critical-modules.txt: single source of truth for the core
  module list. scripts/coverage.sh now reads it (instead of a hardcoded
  string) and update-badges.yml reads the same file, so the badge and
  the `critical` report cannot drift.
- update-badges.yml: a `core coverage` step reuses the unit-coverage
  data (every core module is unit-tested, so unit-only is accurate for
  it) and sed-updates the new badge, like the existing ones.
- README: `core coverage 95%` badge linking to ADR 0004 so a reader can
  find out what "core" means.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NkwFXLFff9PYPy4wgVBJp9

docs/decisions/0004-coverage-policy.md

@@ -88,3 +88,9 @@ omit list.
 - PRs #290 (cover the egress adapter), and the coverage-policy PR that
   introduces this record.
 - `.coveragerc`, `scripts/coverage.sh`, `scripts/diff_coverage.py`.
+- `scripts/critical-modules.txt` — the single source of truth for the
+  core-module list; read by both `scripts/coverage.sh` and the
+  `update-badges.yml` "core coverage" badge so they cannot drift.
+- The README carries a `core coverage` badge (auto-updated from that
+  list) — the headline number, distinct from the informational global
+  `coverage` badge.