feat(auth): M19 harness auth tooling — auth.sh checkpoint + per-launch account injection (#47)

Investigation (pi 0.84.4 docs + host auth.json metadata, values never
read): provider stacking is native (one auth.json keyed by provider;
resolution --api-key > auth.json > env > models.json; OAuth auto-refresh).
Multi-account per provider is NOT native -> named-file design:
auth.<account>.json + per-launch injection.

- scripts/auth.sh: status (provider names, credential types, perms,
  env-side names informational — never credential material) and accounts
  (named files, active marker). Exit codes per convention: 3 missing for
  a read, 2 unparseable, 4 file/environment (symlinks refuse).
- scripts/agent.sh --auth <account>: resolves auth.<account>.json and
  exports PI_AUTH_FILE (the existing compose read-only mount source — no
  new plumbing); missing/invalid account refuses pre-container.
- scripts/test-auth.sh: 13 no-Docker cases; core assertion is the safety
  property itself — fixture key/token/env VALUES never reach output.
- Docs: TOOLS.md Auth section, AGENTS.md command surface + suites.

Headless task runs keep the default credential (worker auth selection is
a separate policy decision). Real-host smoke: anthropic/openai-codex
oauth + zai api_key reported, perms 600, no named accounts yet.

Suites 24/90/14/17/13 + verify green. Agreed sequence M16-M19 complete;
M20 owner-gated.

docs/SESSIONS.md

@@ -13,3 +13,4 @@ are never rewritten or removed; corrections are new entries.
 | 2026-09-03 | owner + assistant (conductor) + ms-test collaborator | Skill revisions adjudicated (#44): ms-communications integrated as-authored (owner preamble restructure + collaborator delivery-discipline hunks); ms-conductor collaborator redraft integrated with conductor remediation (step 3 refusal-vs-outage distinction; preserves owner's outage-dispatch intent inside fail-closed canon); TOOLS.md gains release.sh ensure row | skills/ms-communications/SKILL.md, skills/ms-conductor/SKILL.md, docs/TOOLS.md; suites 24/74/14/17 + verify green; unslop clean ×3 |
 | 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 |