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.

scripts/auth.sh

@@ -0,0 +1,88 @@
+#!/usr/bin/env bash
+# auth.sh — pi credential checkpoint (M19): per-provider auth report and
+# named-account listing. Never prints credential material: provider names,
+# credential types, permission bits, and env var NAMES only.
+#
+# Usage:
+#   scripts/auth.sh status     per-provider report (auth.json + env-side names)
+#   scripts/auth.sh accounts   list named account files (auth.<account>.json)
+#
+# The credential file is the compose read-only mount source (PI_AUTH_FILE,
+# default ~/.pi/agent/auth.json). Resolution follows pi: auth.json entries
+# take priority over environment variables.
+#
+# Exit codes: 0 report produced · 2 credential file unparseable ·
+# 3 file/dir missing for a read · 4 file/environment problem.
+set -uo pipefail
+cd "$(dirname "$0")/.."
+
+AUTH_FILE="${PI_AUTH_FILE:-$HOME/.pi/agent/auth.json}"
+AUTH_DIR="$(dirname "$AUTH_FILE")"
+OP="${1:-}"
+
+case "$OP" in
+    status)
+        if [ ! -e "$AUTH_FILE" ]; then
+            echo "auth: credential file not found: $AUTH_FILE" >&2
+            exit 3
+        fi
+        if [ ! -f "$AUTH_FILE" ] || [ -L "$AUTH_FILE" ]; then
+            echo "auth: credential file must be a regular, non-symbolic-link file: $AUTH_FILE" >&2
+            exit 4
+        fi
+        if ! node -e '
+const fs = require("fs");
+const file = process.argv[1];
+let doc;
+try { doc = JSON.parse(fs.readFileSync(file, "utf8")); }
+catch (e) { console.error("auth: credential file is not valid JSON: " + e.message); process.exit(2); }
+if (typeof doc !== "object" || doc === null || Array.isArray(doc)) {
+  console.error("auth: credential file must be a JSON object keyed by provider"); process.exit(2);
+}
+const st = fs.statSync(file);
+console.log("auth file: " + file + " (perms " + String(st.mode & 0o777).toString(8).padStart(3, "0") + ")");
+const keys = Object.keys(doc).sort();
+if (keys.length === 0) console.log("providers: (none in file)");
+for (const k of keys) {
+  const e = doc[k];
+  let type = "unknown";
+  if (typeof e === "object" && e !== null && typeof e.type === "string" && /^[a-z0-9_-]+$/i.test(e.type)) type = e.type;
+  console.log("  " + k + "  type=" + type + "  source=auth.json");
+}
+console.log("resolution: auth.json entries take priority over environment (pi order: --api-key > auth.json > env > models.json)");
+' "$AUTH_FILE"; then
+            exit 2
+        fi
+        ENV_NAMES="$(env | grep -oE '^[A-Z0-9_]+_API_KEY' | sort -u | paste -sd, -)"
+        if [ -n "$ENV_NAMES" ]; then
+            echo "env-side credential-like names set (informational; file entries take priority): $ENV_NAMES"
+        else
+            echo "env-side credential-like names set: (none)"
+        fi
+        ;;
+    accounts)
+        if [ ! -d "$AUTH_DIR" ]; then
+            echo "auth: directory not found: $AUTH_DIR" >&2
+            exit 3
+        fi
+        shopt -s nullglob
+        FILES=("$AUTH_DIR"/auth.*.json)
+        shopt -u nullglob
+        if [ "${#FILES[@]}" -eq 0 ]; then
+            echo "accounts: (none) — named accounts are auth.<account>.json beside the credential file"
+            exit 0
+        fi
+        echo "named accounts in $AUTH_DIR:"
+        for f in "${FILES[@]}"; do
+            name="$(basename "$f" .json)"
+            name="${name#auth.}"
+            marker=""
+            [ "$f" = "$AUTH_FILE" ] && marker="  <- active (PI_AUTH_FILE)"
+            echo "  $name$marker"
+        done
+        ;;
+    *)
+        echo "usage: scripts/auth.sh status | accounts" >&2
+        exit 4
+        ;;
+esac