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

alembic/versions/0121_agent_run_steps.py

@@ -0,0 +1,51 @@
+"""Create automation_agent_run_steps table for ReAct loop step tracking.
+
+Revision ID: 0121
+Revises: 0120
+"""
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import JSONB, UUID as PGUUID
+
+revision = "0121"
+down_revision = "0120"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "automation_agent_run_steps",
+        sa.Column("id", PGUUID(as_uuid=True), primary_key=True),
+        sa.Column("tenant_id", PGUUID(as_uuid=True), nullable=False, index=True),
+        sa.Column(
+            "agent_run_id",
+            PGUUID(as_uuid=True),
+            sa.ForeignKey("automation_agent_runs.id", ondelete="CASCADE"),
+            nullable=False,
+            index=True,
+        ),
+        sa.Column("step_number", sa.Integer, nullable=False),
+        sa.Column("thought", sa.Text, nullable=True),
+        sa.Column("action", sa.String(255), nullable=True),
+        sa.Column("action_input", JSONB, nullable=True),
+        sa.Column("observation", sa.Text, nullable=True),
+        sa.Column("cost_usd", sa.Float, nullable=False, server_default="0.0"),
+        sa.Column(
+            "created_at",
+            sa.DateTime(timezone=True),
+            nullable=False,
+            server_default=sa.func.now(),
+        ),
+    )
+    op.create_index(
+        "ix_agent_run_steps_run",
+        "automation_agent_run_steps",
+        ["tenant_id", "agent_run_id"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_agent_run_steps_run", table_name="automation_agent_run_steps")
+    op.drop_table("automation_agent_run_steps")