feat(agent-loop): open every prompt with the harness identity section

A static harness:identity section at order -100 — the first occupant of
the documented negative band — states that the agent is powered by the
DeepSeek Harness SDK before the deployment's persona renders. Harness
attribution is a harness fact: it lives on the loop plugin, not in each
deployment's persona, so every agent (subagents included) carries it and
no YAML can forget it. A deployment that must drop it can remove the
section in the system-prompt/assemble waterfall.

Order-band docs updated in all five homes (PromptSection JSDoc, the
system-prompt and agent-loop READMEs, architecture.md, the RFC).

packages/core/agent-loop/README.md

@@ -33,7 +33,7 @@ interface Config {
 }
 ```
 
-Agents listed in config are auto-created at startup. The plugin also registers the per-agent prompt pieces on `ctx.systemPrompt`: the `agent:persona` section (order 0 — `AgentOptions.systemPrompt` renders before all tool guidance) and the built-in `model`/`cwd` prompt variables, each resolved per step from the `assemble({ agent })` context.
+Agents listed in config are auto-created at startup. The plugin also registers the harness-owned prompt pieces on `ctx.systemPrompt`: the `harness:identity` section (order −100 — every agent's prompt opens by stating it is powered by the DeepSeek Harness SDK), the `agent:persona` section (order 0 — `AgentOptions.systemPrompt` renders after it, before all tool guidance), and the built-in `model`/`cwd` prompt variables, resolved per step from the `assemble({ agent })` context.
 
 ### Classes
 

packages/core/agent-loop/src/index.ts

@@ -82,13 +82,20 @@ export class AgentLoop extends Service implements AgentFactory {
     // Provide the agent-creation factory to the registry (effect-scoped: the
     // slot is cleared on dispose).
     ctx.effect(() => this.ctx.agents.setFactory(this), 'agentLoop.setFactory()')
-    // The per-agent prompt pieces, registered once and resolved per assembly
-    // from the AssembleContext the loop passes (loop.ts assembles with
-    // `{ agent }` each step). The persona is the order-0 section — identity
-    // renders before all tool guidance; `{{model}}`/`{{cwd}}` are the built-in
-    // prompt variables projecting the agent's configured model and its
-    // session workspace. A provider returns undefined when the fact is absent
-    // (renderPrompt then rejects a persona that claims it — fail loud).
+    // The prompt pieces the harness itself owns, registered once. The
+    // harness-identity section states what every agent on this loop IS,
+    // ahead of everything (order −100 — before the deployment's persona);
+    // the persona is the order-0 section resolved per assembly from the
+    // AssembleContext the loop passes (loop.ts assembles with `{ agent }`
+    // each step); `{{model}}`/`{{cwd}}` are the built-in prompt variables
+    // projecting the agent's configured model and its session workspace. A
+    // provider returns undefined when the fact is absent (renderPrompt then
+    // rejects a persona that claims it — fail loud).
+    ctx.systemPrompt.section({
+      name: 'harness:identity',
+      order: -100,
+      text: 'You are an AI agent powered by the DeepSeek Harness SDK.',
+    })
     ctx.systemPrompt.section({
       name: 'agent:persona',
       order: 0,

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

@@ -141,7 +141,7 @@ describe('agent loop', () => {
       .toEqual({ diffs: [{ path: 'a.txt', oldText: null, newText: 'x' }] })
   })
 
-  it('renders the persona as the order-0 section — before tool guidance — with {{variables}} resolved', async () => {
+  it('renders harness identity, then the persona, then tool guidance — with {{variables}} resolved', async () => {
     const adapter = new MockAdapter([textResponse('ok')])
     const ctx = await harness(adapter)
     ctx.systemPrompt.section({ name: 'tool:noop', order: 100, text: 'Use the noop tool wisely.' })
@@ -161,7 +161,7 @@ describe('agent loop', () => {
     await waitForIdle(ctx, agent)
 
     const request = adapter.requests[0]
-    expect(request!.system).toBe('You are a test agent on mock.\n\nUse the noop tool wisely.')
+    expect(request!.system).toBe('You are an AI agent powered by the DeepSeek Harness SDK.\n\nYou are a test agent on mock.\n\nUse the noop tool wisely.')
     expect(request!.tools?.map(t => t.name)).toEqual(['noop'])
   })
 
@@ -179,7 +179,7 @@ describe('agent loop', () => {
     send(agent, 'hi')
     await waitForIdle(ctx, agent)
 
-    expect(adapter.requests[0]!.system).toBe('Working in /work/space.')
+    expect(adapter.requests[0]!.system).toBe('You are an AI agent powered by the DeepSeek Harness SDK.\n\nWorking in /work/space.')
   })
 
   it('contains a strict-variable render failure: the turn errors, the loop keeps serving turns', async () => {
@@ -212,7 +212,7 @@ describe('agent loop', () => {
     await waitForIdle(ctx, agent)
 
     expect(adapter.requests).toHaveLength(1)
-    expect(adapter.requests[0]!.system).toBe('In /rescued.')
+    expect(adapter.requests[0]!.system).toBe('You are an AI agent powered by the DeepSeek Harness SDK.\n\nIn /rescued.')
     const turnEnds = agent.session.events.filter(e => e.type === 'turn/end')
     expect(turnEnds).toHaveLength(2)
     expect(turnEnds[1]?.type === 'turn/end' && turnEnds[1].data.reason.kind).toBe('completed')
@@ -426,10 +426,12 @@ describe('agent loop', () => {
     send(agent, 'go')
     await waitForIdle(ctx, agent)
 
-    // One fire per step, in order, each with the assembled system prompt.
+    // One fire per step, in order, each with the assembled system prompt
+    // (here just the loop's own harness-identity section — no persona set).
+    const HARNESS = 'You are an AI agent powered by the DeepSeek Harness SDK.'
     expect(fires).toEqual([
-      { turn: 1, step: 1, fullSystemPrompt: '' },
-      { turn: 1, step: 2, fullSystemPrompt: '' },
+      { turn: 1, step: 1, fullSystemPrompt: HARNESS },
+      { turn: 1, step: 2, fullSystemPrompt: HARNESS },
     ])
   })