feat(skills): add 3-way adversarial plan-challenge phase to reflection orchestrators

After a plan/reflection is elaborated and before it executes, three fresh blind
sub-agents (correctness / robustness / simplicity) attack it on the big model;
the main loop RE-THINKS every aspect a BLOCKER lands (a named plan change, or
[deferred]) and re-challenges once if the plan materially changed. Advisory into
each skill's existing human gate — the human stays the decider.

- lib/challenge-plan.md — reusable phase: fail-safe (never fail open),
  severity-driven (any single-lens BLOCKER = must-address), RE-THINK loop
- agents/plan-challenger.md — challenger role (read-only, big-model per BDR-066)
- lib/tests/plan-challenger.test.sh — 41-assertion structure lock
- wired into 11 orchestrators: ship-feature/init-project/feat/bugfix (build-plan),
  onboard/audit-delta/code-clean (proposals), seo/geo/harden/web-validate (fix-bundle)

Hardened by dogfooding: 3 blind challengers reviewed this feature's own v1 plan
and caught 4 BLOCKERs (fail-open, consensus-buries-lone-finding, wrong model
tier vs BDR-066, false on-disk-plan premise) — all fixed here.

skills/feat/SKILL.md

@@ -117,6 +117,17 @@ PLAN:
 If the approach is ambiguous: ask the user ONE focused question BEFORE
 dispatching — never after (the executor cannot relay questions).
 
+## STEP 1b — CHALLENGE THE PLAN (before branching)
+The STEP 1 plan is a reflection worth attacking before a branch is spent on it.
+Persist it to `.claude/tasks/plans/<date>-<slug>-<HHMM>.md`, then run
+`$HOME/.claude/lib/challenge-plan.md` with `PLAN` = that file, `KIND` = `build-plan`,
+`SCOPE` = the STEP 1 files, `CONSTRAINTS` = the STEP 0.6 in-force BDR/LRN dispositions.
+Three blind challengers attack it; RE-THINK every aspect a BLOCKER lands (a named
+plan change, or `[deferred]`), re-challenge once if the plan materially changed. The
+STEP 3 executor receives the REVISED plan. Before dispatch, print a CHALLENGE SUMMARY
+(BLOCKERs addressed / deferred / lenses returned), surfacing any deferred BLOCKER via
+STEP 1's one-question gate.
+
 ## STEP 2 — BRANCH
 
 **Gitflow aiguillage (before dispatch):** follow `$HOME/.claude/lib/gitflow-aiguillage.md`