feat(coder): orchestrator advanced flow patterns

- TriggerRule type (all_success/one_success/all_done) for parallel deps
- Variable substitution ($stepId.output.field) in agent step prompts
- Approval gate step kind (pauses flow via permission frames)
- flow_step_events table for append-only event-sourced step log
- evaluateTriggerRule pure function in flow-runner-decisions

apps/coder/src/schema.sql

@@ -427,3 +427,14 @@ CREATE INDEX IF NOT EXISTS cross_examinations_battle_idx ON cross_examinations(b
 -- TokenScope: per-category token breakdown on arena contestants and tasks.
 ALTER TABLE contestants ADD COLUMN IF NOT EXISTS token_breakdown JSONB;
 ALTER TABLE tasks ADD COLUMN IF NOT EXISTS token_breakdown JSONB;
+
+-- Orchestrator flow step events (append-only event log for resume/replay).
+CREATE TABLE IF NOT EXISTS flow_step_events (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  run_id UUID NOT NULL REFERENCES flow_runs(id),
+  step_id VARCHAR(64) NOT NULL,
+  event VARCHAR(32) NOT NULL,
+  payload JSONB,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT clock_timestamp()
+);
+CREATE INDEX IF NOT EXISTS flow_step_events_run_idx ON flow_step_events(run_id);