Harness spec 3.1.0: the prefix stays still, in the CLI

The system prompt is made once per session (start, compaction, model
switch), kept in session_meta and resent byte for byte; it holds no
date. What changes mid-session -- the date, a mode the person chose, a
plan or memory or instruction file they edited, a personality, MCP
instructions that arrived later -- is a <session-note> on the next user
message, stored with it. The tool list is fixed for the session:
plan_submit is always offered and answers unavailable outside plan
mode, the wrap-up keeps the tools and runs no call, a withdrawn tool
stays listed and answers its refusal; the first request waits up to
mcp_wait_ms for MCP servers. Compaction asks for its summary as the
next turn of the same conversation. Cached input tokens are parsed
(incl. llama.cpp timings), stored per request and shown in /context.

Spec: context.md §7/§10, loop.json context.note (+ plan_ended,
plan_own, personal, empty, mcp_wait_ms), modes.json plan.refused,
compact.md without the transcript heading. Conformance area prefix.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/harness.test.ts

@@ -206,7 +206,7 @@ test("every {{variable}} in a description is declared, and none reaches a model"
   const plan = normalise({ title: "T", phases: [{ title: "P", tasks: ["a"] }] }) as Plan
   for (const mode of MODES) {
     for (const p of [undefined, plan]) {
-      const system = app.engine.o.system(mode, app.engine.model, p)
+      const system = app.engine.compose(mode, p)
       expect(system).not.toContain("{{")
       expect(system).toContain("Use the instructions below and the tools you have to help with software engineering and with running the project.")
       expect(system).toContain("- Your output is shown in a terminal and rendered as GitHub-flavoured markdown in a monospace font. Be concise")
@@ -221,7 +221,7 @@ test("every {{variable}} in a description is declared, and none reaches a model"
     }
   }
   app.setContext({ instructions: "Be brief.", prototypes: { offered: true, sources: "" } }, "example")
-  const withContext = app.engine.o.system("manual", app.engine.model, plan)
+  const withContext = app.engine.compose("manual", plan)
   expect(withContext).not.toContain("{{")
   expect(withContext).toContain("## Instructions from example\n\nBe brief.")
   expect(withContext).toContain("prototype-page")