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.

docs/SESSIONS.md

@@ -14,3 +14,4 @@ are never rewritten or removed; corrections are new entries.
 | 2026-09-03 | assistant (conductor) | M18 seat-role progressive capability restriction (#45): roles/<role>.json contracts (strict schema, name-filename binding, network declared), mosaic-task.mjs resolve-role, agent.sh ceiling intersection with fail-closed refusals, roles/researcher.json shipped, 14 suite cases (task 74 → 88) | scripts/mosaic-task.mjs, scripts/agent.sh, scripts/test-task.sh, roles/researcher.json, docs; suites 24/88/14/17 + verify green |
 | 2026-09-03 | owner (decision + live verification) + assistant (conductor) | M18 live verification + follow-up (#46): owner confirmed narrowing/refusal/tool-free live; seatless launch under AGENTS_DIR override discovered and made fail-closed (exit 4); task suite 88 → 90 | scripts/agent.sh, scripts/test-task.sh, docs/TOOLS.md; suites 24/90/14/17 + verify green |
 | 2026-09-03 | assistant (conductor) | M19 harness auth tooling (#47): pi auth investigation (native provider stacking, no native multi-account), scripts/auth.sh status/accounts (never prints credential material), agent.sh --auth per-launch injection via PI_AUTH_FILE, test-auth.sh suite (13 cases incl. secret-never-printed assertions) | scripts/auth.sh, scripts/agent.sh, scripts/test-auth.sh, docs/TOOLS.md, AGENTS.md; suites 24/90/14/17/13 + verify green |
+| 2026-09-03 | owner (direction) + assistant (conductor) | M19 correction (#48): mosaic-managed auth moved from ~/.pi to the data root (auth/<account>.json, 0600 enforced); ~/.pi read-only to the stack as a ROADMAP standing decision; auth.sh config-driven; test-auth 15 cases | scripts/auth.sh, scripts/agent.sh, scripts/test-auth.sh, docs/TOOLS.md, docs/plans/ROADMAP.md, README.md; suites 24/15/90/14/17 + verify green |