chore: drop dead NEXT_PUBLIC_GATEWAY_URL; refresh Phase P increment status

.env.example: NEXT_PUBLIC_GATEWAY_URL has zero consumers anywhere in
apps/, packages/, or plugins/ — the SPA is origin-relative by design and
the legacy Next tree never reads it. Remove the whole stale Web App
section so new deployments stop configuring a dead variable.

PHASE-P-STRUCTURE.md: increment map was stale — P3 is merged (PR #1151,
repair PR #1154) and P4 is in progress (P4-1 read-only projects + tasks
merged, PR #1153). Re-verify the doc against next head 3bd490c0; add the
P4-1 pages to the tree listing. Blocker #1145 remains open and the P5/P6
rows are unchanged.

webui-audit S1 hygiene items (lane REPORT.md).

.env.example

@@ -40,11 +40,6 @@ BETTER_AUTH_SECRET=change-me-to-a-random-32-char-string
 BETTER_AUTH_URL=http://localhost:14242
 
 
-# ─── Web App (Next.js) ───────────────────────────────────────────────────────
-# Public gateway URL — accessible from the browser, not just the server.
-NEXT_PUBLIC_GATEWAY_URL=http://localhost:14242
-
-
 # ─── OpenTelemetry ───────────────────────────────────────────────────────────
 # OTLP HTTP endpoint (otel-collector or any OpenTelemetry-compatible backend)
 OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4318