feat(F-LOOP): true ReAct loop with structured Thought/Action/Observation step tracking

- app/ai/agent_loop.py: ReActStep + ReActResult dataclasses, run_react_loop()
  with LLM→Tool→Observe loop, max_steps/timeout graceful stop, ErrorCategory
  retry (TRANSIENT→retry, PERMANENT→stop, PARTIAL→continue), cost accumulation,
  agent.step hook, on_step callback
- app/plugins/builtins/automation/models.py: AgentRunStep model
- alembic/versions/0121_agent_run_steps.py: migration for agent run steps table
- app/plugins/builtins/automation/agent_runner.py: refactored to use run_react_loop(),
  saves steps to DB, updates AgentRun with cost/status/duration
- tests/test_agent_loop.py: 11 tests (all passing, mocked, no DB/LLM needed)
- PROGRESS.md: Phase F started, F-LOOP marked done

PROGRESS.md

@@ -15,7 +15,7 @@
 | C.5 — Import/Export | `done` | 2026-08-13 | 2026-08-13 | 8 | 8 |
 | D — Undo/Restore | `done` | 2026-08-13 | 2026-08-13 | 13 | 13 |
 | E — Search | `done` | 2026-08-14 | 2026-08-14 | 24 | 24 |
-| F — Agents | `not_started` | — | — | 0 | ~28 |
+| F — Agents | `in_progress` | 2026-08-17 | — | 1 | ~28 |
 | G — Workflows | `not_started` | — | — | 0 | ~24 |
 | H — Knowledge | `not_started` | — | — | 0 | ~18 |
 | I — Integration & Workstream | `not_started` | — | — | 0 | ~25 |
@@ -237,6 +237,16 @@ Detaillierte Task-Listen werden beim Start der jeweiligen Phase eingetragen. Sie
 
 ---
 
+## Phase F — Agents
+
+### F.1 ReAct Loop
+
+| Task | Status | Forgejo Issue | Verifiziert |
+|------|-------|---------------|------------|
+| F-LOOP | `done` | — | ✅ `app/ai/agent_loop.py` — ReActStep/ReActResult dataclasses + `run_react_loop()` mit LLM→Tool→Observe Loop, max_steps/timeout Graceful-Stop, ErrorCategory-basiertes Retry (TRANSIENT→retry, PERMANENT→stop, PARTIAL→continue), Cost-Accumulation, `agent.step` Hook. `AgentRunStep` Model + Migration 0121. `agent_runner.py` auf ReAct-Loop umgestellt. 11/11 Tests grün (mocked, kein DB/LLM benötigt) |
+
+---
+
 ## Blockierte Tasks
 
 | Task | Grund | Blockiert seit | Lösung |