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/system-prompt/src/index.ts

@@ -54,9 +54,9 @@ export interface PromptSection {
   /** Unique name — a duplicate registration throws (see {@link SystemPrompt.section}). */
   name: string
   /**
-   * Sections are concatenated in ascending order. Convention: `0` is the
-   * per-agent persona, tool guidance uses 100–199; negative orders render
-   * before the persona.
+   * Sections are concatenated in ascending order. Convention: `-100` is the
+   * harness identity, `0` the per-agent persona, tool guidance uses 100–199;
+   * other negative orders also render before the persona.
    */
   order: number
   /**