feat(invariants): add package-owned service seam

packages/core/agent/src/invariant.ts

@@ -0,0 +1,35 @@
+/** Package-owned agent lifecycle invariants. @module @deepseek-ai/dsh-agent/invariant */
+
+import type { Context } from 'cordis'
+import type { InvariantInstaller } from '@deepseek-ai/dsh-invariants'
+import type { Agent, AgentStatus } from '@deepseek-ai/dsh-agent'
+
+const PACKAGE_NAME = '@deepseek-ai/dsh-agent'
+
+/** Cordis companion plugin name. */
+export const name = 'agent-invariant'
+/** Services required before the companion can register. */
+export const inject = ['invariants']
+
+/** Install the agent contribution into its child registration fiber. */
+const install: InvariantInstaller = (ctx, fail) => {
+  const lastStatus = new WeakMap<Agent, AgentStatus>()
+  ctx.on('agent/status', (agent, status) => {
+    const previous = lastStatus.get(agent)
+    if (previous === status) {
+      fail(`agent/status repeated ${status} (no-op transition)`)
+    }
+    if (previous === 'disposed') {
+      fail(`agent/status left terminal state disposed → ${status}`)
+    }
+    lastStatus.set(agent, status)
+  }, { global: true })
+}
+
+/**
+ * Register the agent invariant companion.
+ * @param ctx - Cordis context carrying the invariant service.
+ * @returns the installed registration's disposer after setup succeeds.
+ */
+export const apply = (ctx: Context): Promise<() => void> =>
+  Promise.resolve(ctx.invariants.register(PACKAGE_NAME, install))