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

app/plugins/builtins/automation/models.py

@@ -16,6 +16,7 @@ from sqlalchemy import (
     String,
     Text,
     UniqueConstraint,
+    func,
 )
 from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.dialects.postgresql import UUID as PGUUID
@@ -254,6 +255,34 @@ class AutomationRun(Base, TenantMixin):
     dry_run: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
 
 
+class AgentRunStep(Base, TenantMixin):
+    """Individual step in a ReAct loop execution (Thought → Action → Observation)."""
+
+    __tablename__ = "automation_agent_run_steps"
+    __table_args__ = (
+        Index("ix_agent_run_steps_run", "tenant_id", "agent_run_id"),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    agent_run_id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True),
+        ForeignKey("automation_agent_runs.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    step_number: Mapped[int] = mapped_column(Integer, nullable=False)
+    thought: Mapped[str | None] = mapped_column(Text, nullable=True)
+    action: Mapped[str | None] = mapped_column(String(255), nullable=True)
+    action_input: Mapped[dict[str, Any] | None] = mapped_column(JSONB, nullable=True)
+    observation: Mapped[str | None] = mapped_column(Text, nullable=True)
+    cost_usd: Mapped[float] = mapped_column(Float, nullable=False, default=0.0)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )
+
+
 class AgentSubtask(Base, TenantMixin):
     """A subtask delegated from one agent to another for multi-agent orchestration."""