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/alembic/versions/144_add_automation_tables.py

@@ -125,7 +125,6 @@ def upgrade() -> None:
             error JSONB,
             started_at TIMESTAMP WITH TIME ZONE,
             finished_at TIMESTAMP WITH TIME ZONE,
-            agent_session_id VARCHAR(200),
             created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT NOW()
         );
         """