feat(agent): add durable agent invocations and spawning

TERMINOLOGY.md

@@ -46,6 +46,10 @@ Canonical words used across Junior's code and documentation.
 - **Session record**: the persisted read model for one resumable turn.
 - **Conversation execution**: mutable operational state for a conversation,
   such as mailbox state, worker lease, checkpoints, and activity status.
+- **Agent binding**: a named reference, scoped to one parent agent
+  conversation, that reuses one child conversation and its history.
+- **Agent invocation**: one retry-safe delegated task sent from a parent agent
+  conversation to a child conversation, including its durable terminal result.
 - **Reasoning level**: the configured or selected amount of model reasoning for
   a turn: `none`, `low`, `medium`, `high`, or `xhigh`.
 - **Reply**: a destination-visible message owned by delivery or reply-policy
@@ -54,6 +58,9 @@ Canonical words used across Junior's code and documentation.
 ## Naming Guidance
 
 - Use `turn`, `run`, and `slice` only with the meanings above.
+- Use `agent invocation` for delegated child work; do not shorten it to
+  `invocation` where it could be confused with a model or serverless
+  invocation.
 - Use `message` for platform chat content. Use `user_message`,
   `assistant_message`, and `tool_result` for replayable agent history.
 - Use `agent history item` when referring to those three native event types as

packages/junior/migrations/0012_sour_vargas.sql

@@ -0,0 +1,36 @@
+CREATE TABLE "junior_agent_bindings" (
+	"parent_conversation_id" text NOT NULL,
+	"name" text NOT NULL,
+	"child_conversation_id" text NOT NULL,
+	"reasoning_level" text,
+	CONSTRAINT "junior_agent_bindings_parent_conversation_id_name_pk" PRIMARY KEY("parent_conversation_id","name")
+);
+--> statement-breakpoint
+CREATE TABLE "junior_agent_invocations" (
+	"invocation_id" text PRIMARY KEY NOT NULL,
+	"parent_conversation_id" text NOT NULL,
+	"child_conversation_id" text NOT NULL,
+	"agent_name" text,
+	"input" text NOT NULL,
+	"actor_json" jsonb NOT NULL,
+	"credential_context_json" jsonb,
+	"source_json" jsonb NOT NULL,
+	"destination_json" jsonb NOT NULL,
+	"destination_visibility" text,
+	"reasoning_level" text,
+	"status" text NOT NULL,
+	"mailbox_status" text NOT NULL,
+	"result" text,
+	"error_message" text,
+	"created_at" timestamp with time zone NOT NULL,
+	"updated_at" timestamp with time zone NOT NULL,
+	"terminal_at" timestamp with time zone
+);
+--> statement-breakpoint
+ALTER TABLE "junior_agent_bindings" ADD CONSTRAINT "junior_agent_bindings_parent_conversation_id_junior_conversations_conversation_id_fk" FOREIGN KEY ("parent_conversation_id") REFERENCES "public"."junior_conversations"("conversation_id") ON DELETE no action ON UPDATE no action;--> statement-breakpoint
+ALTER TABLE "junior_agent_bindings" ADD CONSTRAINT "junior_agent_bindings_child_conversation_id_junior_conversations_conversation_id_fk" FOREIGN KEY ("child_conversation_id") REFERENCES "public"."junior_conversations"("conversation_id") ON DELETE no action ON UPDATE no action;--> statement-breakpoint
+ALTER TABLE "junior_agent_invocations" ADD CONSTRAINT "junior_agent_invocations_parent_conversation_id_junior_conversations_conversation_id_fk" FOREIGN KEY ("parent_conversation_id") REFERENCES "public"."junior_conversations"("conversation_id") ON DELETE no action ON UPDATE no action;--> statement-breakpoint
+ALTER TABLE "junior_agent_invocations" ADD CONSTRAINT "junior_agent_invocations_child_conversation_id_junior_conversations_conversation_id_fk" FOREIGN KEY ("child_conversation_id") REFERENCES "public"."junior_conversations"("conversation_id") ON DELETE no action ON UPDATE no action;--> statement-breakpoint
+CREATE UNIQUE INDEX "junior_agent_bindings_child_idx" ON "junior_agent_bindings" USING btree ("child_conversation_id");--> statement-breakpoint
+CREATE INDEX "junior_agent_invocations_child_idx" ON "junior_agent_invocations" USING btree ("child_conversation_id");--> statement-breakpoint
+CREATE INDEX "junior_agent_invocations_mailbox_idx" ON "junior_agent_invocations" USING btree ("mailbox_status","created_at");