docs(skills): tighten debug, root-cause-trace, and five-whys

Three audit-pass fixes across the debugging skills.

debug now captures environment and recent-change context (versions, flags, dataset, seed/clock, concurrency, recent commits) as a Phase-1 step. Many intermittent bugs live in state or environment, not a local code path, and "what changed recently" is often the fastest route to the cause.

root-cause-trace's defense-in-depth said to add a check at every layer that could have caught the bad value, which breeds validation spam. It now adds checks only at boundary-owning layers (ingress, persistence, the invariant owner, final render), and says a pass-through function that owns neither a boundary nor an invariant shouldn't get a duplicate null check.

five-whys now makes each link carry an evidence field and a counterfactual: if you remove this cause, does the symptom above still happen? That's the guard against a tidy chain that reads well but wouldn't have prevented the failure.

1 files changed, 6 insertions, 5 deletions

diff --git a/root-cause-trace/SKILL.md b/root-cause-trace/SKILL.md
index fad4601..bb46f41 100644
--- a/root-cause-trace/SKILL.md
+++ b/root-cause-trace/SKILL.md
@@ -99,13 +99,14 @@ Two actions, in order:
 
 **a. Fix at the trigger.** In the example: change the query to `INNER JOIN`, or explicitly handle customer-less orders (depending on intent).
 
-**b. Add defense-in-depth.** For each layer between the trigger and the symptom, ask: *could this layer have caught the bad value?* If yes, add the check.
+**b. Add defense-in-depth — at boundaries, not everywhere.** Don't armor every layer between the trigger and the symptom; that's validation spam that buries the real check and slows every call. Add a check only at a layer that *owns* a boundary or invariant:
 
-- Parser/validator layer: reject rows without `customer_id`
-- Service layer: throw if `order.customer` is nil instead of passing it downstream
-- Formatter layer: render "Unknown customer" rather than crashing
+- **Ingress / trust boundary** — where untrusted or external data first enters (parser/validator layer: reject rows without `customer_id`)
+- **Persistence boundary** — before writing to or reading from a store
+- **Invariant-owning layer** — the service that's supposed to guarantee a fact (throw if `order.customer` is nil rather than passing it downstream)
+- **Final render/output** — degrade gracefully (render "Unknown customer" rather than crashing)
 
-Each defense means the next time something similar happens, it surfaces earlier and with better context. The goal isn't any single check — it's that the bad value can't propagate silently.
+A pass-through function that neither owns the invariant nor crosses a boundary should *not* get a duplicate null check — let the boundary layers carry it. Each boundary defense means a similar bad value surfaces earlier and with better context; the goal isn't any single check, it's that the bad value can't propagate silently past the layer responsible for it.
 
 ## Adding Instrumentation