fix(auth): mosaic-managed auth lives under the data root, never ~/.pi (#48)

Owner direction: the stack must never impact default harness usage.
Correction to M19 as shipped (nothing had been created in ~/.pi — the
move breaks nothing).

- Mosaic-managed accounts: <dataRoot>/auth/<account>.json, perms 0600
  enforced (loose perms flagged in listings, refused by --auth — mirrors
  gitea-api.sh credential hygiene).
- ~/.pi is read-only to the stack, permanently; the only interaction
  remains the existing read-only container mount of the default
  credential. Recorded as a ROADMAP standing decision.
- auth.sh is now config-driven (data root from config.json, fail closed,
  consistent with every other tool); status reports both sources labeled.
- agent.sh --auth resolution moved after load_config (needs the data
  root); missing/symlinked/non-0600 accounts refuse.
- test-auth.sh: 15 no-Docker cases (accounts-create-nothing, loose-perms
  refusal, invalid-config refusal added). Test-authoring correction
  recorded in BUILD-LOG (fixture-state mismatch caught before running).

Suites 24/15/90/14/17 + verify green.

BUILD-LOG.md

@@ -553,3 +553,31 @@ Auth is checkpointable without exposing credentials, and multi-account has
 a per-launch path. Suites 24/90/14/17/13 + verify green. The agreed
 ROADMAP sequence M16–M19 is complete; M20 (packages/* restructure +
 unified CLI) is owner-gated.
+
+## Phase 24 follow-up: auth ownership corrected — data root, never ~/.pi (#48)
+
+- Owner correction after M19 review: mosaic-managed named accounts must
+  not live inside ~/.pi — the stack must never impact default harness
+  usage. Recorded as a ROADMAP standing decision: the stack never writes
+  to default harness config locations; ~/.pi is read-only to the stack.
+- Correction noted honestly: M19 as shipped placed accounts beside
+  ~/.pi/agent/auth.json. Nothing had been created there (accounts reported
+  none), so the move breaks nothing.
+- auth.sh is now config-driven (data root from config.json, fail closed —
+  consistent with every other tool); status reports both sources labeled:
+  default harness credential (read-only to the stack) + mosaic-managed
+  accounts.
+- Accounts live at <dataRoot>/auth/<account>.json, perms 0600 enforced:
+  loose perms flagged in listings and refused by agent.sh --auth
+  (mirrors scripts/gitea-api.sh credential hygiene).
+- Test-authoring correction: the first suite rewrite asserted "(none)" in
+  cases whose fixtures had already created accounts; caught on review
+  before any run, assertions rewritten to match fixture state.
+- auth.sh suite: 13 → 15 cases (accounts-create-nothing, loose-perms
+  refusal, invalid-config refusal added).
+
+## Result
+
+Suites 24/15/90/14/17 + verify green. Default harness usage is untouched
+by design; all mosaic-managed credentials live inside the governed data
+root.