db: agent enrollment schema (M4-4a, migration 0021)

Additive-only DDL per the merged design (docs/plans/
2026-08-29-agent-enrollment-command-design.md SS4): agents gains
nullable harness + enrolled_at (no backfill); new agent_audit_events
(append-only, seq identity, FK-free agent linkage, causation self-FK,
closed event-type CHECK), agent_outbox (hierarchy shape, one row per
event, closed status enum), agent_idempotency_fence (contract 3 SS4.3
shape: UNIQUE (operation, key), replay-mode CHECK, FK-free outcome
reference).

Schema-level witnesses in agent-enrollment.witness.test.ts (PGlite +
real-PostgreSQL legs): nullable enrollment columns, event-type CHECK,
per-agent seq ordering, deletion-safe linkage (events and fence rows
survive agent deletion; information_schema FK probes), causation and
outbox RESTRICT, fence uniqueness and mode CHECK. Command-level
witnesses land with M4-4b.

packages/db/drizzle/0021_agent_enrollment.sql

@@ -0,0 +1,47 @@
+CREATE TYPE "public"."agent_outbox_status" AS ENUM('pending', 'processing', 'delivered');--> statement-breakpoint
+CREATE TABLE "agent_audit_events" (
+	"id" uuid PRIMARY KEY DEFAULT gen_random_uuid() NOT NULL,
+	"seq" bigint GENERATED ALWAYS AS IDENTITY (sequence name "agent_audit_events_seq_seq" INCREMENT BY 1 MINVALUE 1 MAXVALUE 9223372036854775807 START WITH 1 CACHE 1),
+	"event_type" text NOT NULL,
+	"actor_id" text NOT NULL,
+	"agent_id" uuid NOT NULL,
+	"correlation_id" text NOT NULL,
+	"causation_id" uuid,
+	"payload" jsonb NOT NULL,
+	"created_at" timestamp with time zone DEFAULT now() NOT NULL,
+	CONSTRAINT "agent_audit_events_type_check" CHECK (event_type IN ('agent.enrolled', 'agent.enrollment.replayed'))
+);
+--> statement-breakpoint
+CREATE TABLE "agent_idempotency_fence" (
+	"id" uuid PRIMARY KEY DEFAULT gen_random_uuid() NOT NULL,
+	"operation" text NOT NULL,
+	"idempotency_key" text NOT NULL,
+	"actor_id" text NOT NULL,
+	"authorization_scope" text NOT NULL,
+	"payload_digest" text NOT NULL,
+	"replay_mode" text DEFAULT 'actor-bound' NOT NULL,
+	"outcome_agent_id" uuid NOT NULL,
+	"created_at" timestamp with time zone DEFAULT now() NOT NULL,
+	CONSTRAINT "agent_idempotency_fence_replay_mode_check" CHECK (replay_mode IN ('actor-bound', 'shared'))
+);
+--> statement-breakpoint
+CREATE TABLE "agent_outbox" (
+	"id" uuid PRIMARY KEY DEFAULT gen_random_uuid() NOT NULL,
+	"event_id" uuid NOT NULL,
+	"correlation_id" text NOT NULL,
+	"status" "agent_outbox_status" DEFAULT 'pending' NOT NULL,
+	"created_at" timestamp with time zone DEFAULT now() NOT NULL,
+	"updated_at" timestamp with time zone DEFAULT now() NOT NULL,
+	"delivered_at" timestamp with time zone
+);
+--> statement-breakpoint
+ALTER TABLE "agents" ADD COLUMN "harness" text;--> statement-breakpoint
+ALTER TABLE "agents" ADD COLUMN "enrolled_at" timestamp with time zone;--> statement-breakpoint
+ALTER TABLE "agent_audit_events" ADD CONSTRAINT "agent_audit_events_causation_id_agent_audit_events_id_fk" FOREIGN KEY ("causation_id") REFERENCES "public"."agent_audit_events"("id") ON DELETE restrict ON UPDATE no action;--> statement-breakpoint
+ALTER TABLE "agent_outbox" ADD CONSTRAINT "agent_outbox_event_id_agent_audit_events_id_fk" FOREIGN KEY ("event_id") REFERENCES "public"."agent_audit_events"("id") ON DELETE restrict ON UPDATE no action;--> statement-breakpoint
+CREATE UNIQUE INDEX "agent_audit_events_seq_idx" ON "agent_audit_events" USING btree ("seq");--> statement-breakpoint
+CREATE INDEX "agent_audit_events_agent_seq_idx" ON "agent_audit_events" USING btree ("agent_id","seq");--> statement-breakpoint
+CREATE INDEX "agent_audit_events_correlation_idx" ON "agent_audit_events" USING btree ("correlation_id");--> statement-breakpoint
+CREATE UNIQUE INDEX "agent_idempotency_fence_operation_key_idx" ON "agent_idempotency_fence" USING btree ("operation","idempotency_key");--> statement-breakpoint
+CREATE UNIQUE INDEX "agent_outbox_event_idx" ON "agent_outbox" USING btree ("event_id");--> statement-breakpoint
+CREATE INDEX "agent_outbox_status_created_idx" ON "agent_outbox" USING btree ("status","created_at");

packages/db/drizzle/meta/_journal.json

@@ -148,6 +148,13 @@
       "when": 1787963521142,
       "tag": "0020_special_betty_brant",
       "breakpoints": true
+    },
+    {
+      "idx": 21,
+      "version": "7",
+      "when": 1788053011351,
+      "tag": "0021_agent_enrollment",
+      "breakpoints": true
     }
   ]
 }