docs: add AGENTS.md + authorization enforcement gates

NEW: AGENTS.md — complete agent workflow documentation
- Step-by-step PR workflow for all agents
- Authorization matrix (who can change which contracts)
- Emergency bypass procedure
- Quick start commands

NEW: scripts/prose-auth-check.sh — authorization enforcement
- Blocks unauthorized agents from changing CRITICAL contracts
- infrastructure-control, proxmox-monitor: abiba only
- hermes-config-template: abiba, mumuni, tanko
- zulip-health: abiba, mumuni
- All scripts: abiba only
- Fails CI if unauthorized changes detected

UPDATED: .gitea/workflows/pr-pipeline.yaml
- Added Stage 0: auth check (runs first)
- Added gate job that confirms all 4 checks passed
- Expanded trigger paths to include scripts/*.sh

UPDATED: branch protection — now requires 4 contexts:
  pr-pipeline / auth, validate, lint, ai-review

scripts/prose-auth-check.sh

@@ -0,0 +1,68 @@
+#!/bin/bash
+# prose-auth-check.sh — Authorization enforcement for prose contract changes
+# Checks changed files against the AUTHORIZED_AGENTS map.
+# Fails if an unauthorized agent changes a restricted contract.
+set -euo pipefail
+
+echo "╔═══════════════════════════════════╗"
+echo "║   Authorization Enforcement       ║"
+echo "╚═══════════════════════════════════╝"
+echo ""
+
+# Authorized agents for restricted contracts
+# Format: contract_pattern|authorized_agents (comma-separated)
+declare -A RESTRICTED
+RESTRICTED["infrastructure-control.prose.md"]="abiba"
+RESTRICTED["proxmox-monitor.prose.md"]="abiba"
+RESTRICTED["hermes-config-template.prose.md"]="abiba,mumuni,tanko"
+RESTRICTED["zulip-health.prose.md"]="abiba,mumuni"
+RESTRICTED["scripts/pm2-self-heal.sh"]="abiba"
+RESTRICTED["scripts/prose-lint.sh"]="abiba"
+RESTRICTED["scripts/prose-ai-review.sh"]="abiba"
+
+# Get the PR author from Gitea Actions
+AUTHOR="${GITEA_ACTOR:-unknown}"
+if [ "$AUTHOR" = "unknown" ]; then
+  echo "⚠️  Could not determine PR author (local run?). Skipping auth check."
+  exit 0
+fi
+
+echo "PR author: $AUTHOR"
+echo ""
+
+# Get changed files from the PR
+CHANGED=$(git diff --name-only origin/main...HEAD 2>/dev/null || git diff --name-only HEAD~1 2>/dev/null || echo "")
+
+if [ -z "$CHANGED" ]; then
+  echo "No changed files to check."
+  exit 0
+fi
+
+FAILED=0
+
+for file in $CHANGED; do
+  # Check if this file is restricted
+  for pattern in "${!RESTRICTED[@]}"; do
+    if [[ "$file" == *"$pattern"* ]]; then
+      ALLOWED="${RESTRICTED[$pattern]}"
+      if echo "$ALLOWED" | grep -qw "$AUTHOR"; then
+        echo "  ✅ $file — $AUTHOR is authorized"
+      else
+        echo "  🚫 $file — $AUTHOR is NOT authorized (allowed: $ALLOWED)"
+        FAILED=1
+      fi
+      break
+    fi
+  done
+done
+
+echo ""
+if [ $FAILED -eq 1 ]; then
+  echo "❌ AUTHORIZATION FAILED — unauthorized agent attempted to change restricted contracts"
+  echo ""
+  echo "If this is an emergency fix, add [EMERGENCY] to the PR title."
+  echo "Otherwise, ask an authorized agent ($ALLOWED) to make this change."
+  exit 1
+else
+  echo "✅ Authorization check passed"
+fi