refactor(system-prompt): rename TOOL_ORDER_REST from '...' to '<unlisted-tools>'

A three-dot rest entry reads as elision in a cordis.yml; the spelled-out
sentinel says what lands there. The literal now appears once in code (the
constant) and once in the value-pinning test; every other reference — the
forwarding tests included — imports TOOL_ORDER_REST, which adds the
dsh-system-prompt devDependency to the two app packages.

Review follow-up on #196.

packages/core/agent-core/tests/agent-core.spec.ts

@@ -1,6 +1,7 @@
 import { describe, expect, it } from 'vitest'
 import { Context } from 'cordis'
 import Loader from '@cordisjs/plugin-loader'
+import { TOOL_ORDER_REST } from '@deepseek-ai/dsh-system-prompt'
 import * as agentCore from '../src/index.ts'
 import { AgentId } from '@deepseek-ai/dsh-agent'
 
@@ -68,7 +69,7 @@ describe('dsh-agent-core bundle', () => {
   })
 
   it('forwards toolOrder to the system-prompt assembly', async () => {
-    const ctx = await mount({ toolOrder: ['zulu', '...'] })
+    const ctx = await mount({ toolOrder: ['zulu', TOOL_ORDER_REST] })
     // The bundle's own bash tools pend on the absent `ctx.bash` executor in
     // this providerless mount, so register two plain tools to order.
     for (const name of ['alpha', 'zulu']) {