refactor(automation): drop agent_session_id from AutomationRun

A run can contain zero, one, or N agent_task steps. A single
agent_session_id at the run level holds at most one of them, so the
column is the wrong shape for the data.

Per-step session ids (LangGraph thread/checkpoint reference for an
agent_task step) live inside step_results[i] alongside the rest of
the per-step bag (status, timings, output). Each agent step records
its own; non-agent steps record nothing. Run-level "primary session"
is a UI concern, not a schema concern.

Trade-off: trace -> run reverse lookup is now a JSONB query, not an
index hit. Usually traversal goes run -> trace; if the reverse
becomes hot we add a GIN index on step_results or a generated
column — both additive.

Changes:
- AutomationRun: drop the agent_session_id column; module docstring
  notes where per-step session ids now live.
- Migration 144: drop the column from the CREATE TABLE; downgrade
  unchanged.

Safe to edit migration 144 in place (vs. add 145 with ALTER ... DROP):
this branch has not shipped and the table has never existed in any
deployed database.

surfsense_backend/app/automations/persistence/models/run.py

@@ -1,4 +1,9 @@
-"""``automation_runs`` table — immutable per-fire execution record."""
+"""``automation_runs`` table — immutable per-fire execution record.
+
+Per-step metadata (incl. any LangGraph session id for an ``agent_task`` step)
+lives inside ``step_results[i]``, since a single run may contain zero, one,
+or N agent steps.
+"""
 
 from __future__ import annotations
 
@@ -8,7 +13,6 @@ from sqlalchemy import (
     Enum as SQLAlchemyEnum,
     ForeignKey,
     Integer,
-    String,
 )
 from sqlalchemy.dialects.postgresql import JSONB
 
@@ -54,5 +58,3 @@ class AutomationRun(BaseModel, TimestampMixin):
 
     started_at = Column(TIMESTAMP(timezone=True), nullable=True)
     finished_at = Column(TIMESTAMP(timezone=True), nullable=True)
-
-    agent_session_id = Column(String(200), nullable=True)