v0.5.1 — end-to-end gap-sweep fixes

Final end-to-end gap sweep — the install→introspect→render→hooks→workflow spine was
confirmed clean, but running the WHOLE kit once on ordinary input surfaced scaffolder
bugs. All fixed.

Fixed

• ADR / spec title with / or & crashed or corrupted the scaffolder.
  new-adr.sh / new-spec.sh filled the template with sed, so a perfectly ordinary
  title — CI/CD, A/B test, Q&A — hit sed: bad flag (crash + a 0-byte ADR
  occupying the number) or silently mangled the &. Now filled with python3
str.replace (metacharacter-safe). The most likely first-run failure, gone.
• new-adr.sh used ls | grep (SC2010, fragile on odd filenames) → replaced with a
  glob loop. CI shellcheck now covers ALL scripts (the scaffolders were unchecked)
  and is blocking (was || true).
• render.sh orphaned a stale <old-stack>-architect.md when the stack slug changed
  on re-run (e.g. typescript → node) — now reaped, like db-verify/ui-verify.
• detect.sh silently failed (empty stdout, exit 0) when python3 was missing — now
  emits {"error":"python3 not found"} and exits non-zero.
• README test-count claim made version-agnostic (was a stale "83").

Added

• tests/resume_loop_test.sh pins the handoff ↔ context.md ↔ pickup marker contract (a
  1-char drift in any of the three now fails CI instead of silently breaking resume).
• Regression tests for the metacharacter titles + the architect reap. → 172 tests.