feat: replace agent_loop.py with agentloop

Switch from the bespoke 1136-line Python orchestrator to the community
agentloop tool (https://github.com/guettli/agentloop). The new tool
handles the issue → agent → PR pipeline via a label state machine using
loop/plan and loop/code labels, running every 5 minutes via cron.

Removes: scripts/agent_loop.py, scripts/test_agent_loop.py
Removes: .forgejo/workflows/monitor.yml (no heartbeat concept in agentloop)
Updates: AGENTS.md to document the new loop/ label workflow

agentloop config lives in ~/agentloop/loop/sharedinbox/ on the host.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.forgejo/workflows/monitor.yml

@@ -1,18 +0,0 @@
-name: Monitor Agent Loop
-
-on:
-  schedule:
-    - cron: '0 */2 * * *'  # every 2 hours
-  workflow_dispatch:
-
-jobs:
-  monitor:
-    name: Check Agent Loop Health
-    runs-on: ubuntu-latest
-    timeout-minutes: 5
-
-    steps:
-      - uses: actions/checkout@v4
-
-      - name: Check agent loop heartbeat
-        run: python3 scripts/agent_loop.py monitor