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mode, the wrap-up keeps the tools and runs no call, a withdrawn tool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/subagent.test.ts

@@ -63,7 +63,8 @@ describe("subagents", () => {
     const child = fake!.requests[1]
     expect(child.messages[0].content).toContain('You are the "explore" subagent')
     expect(child.messages[0].content).not.toContain("Permission mode: plan")
-    expect(child.messages.at(-1).content).toBe("Find why average is wrong in calc.py")
+    // Its prompt has no date either: its first message says it (harness context.md §10).
+    expect(child.messages.at(-1).content).toMatch(/^Find why average is wrong in calc\.py\n<session-note>\nToday is \d{4}-\d{2}-\d{2}\.\n<\/session-note>$/)
     // its own job tools for commands it starts, and report for a word to the main agent
     expect(names(child)).toEqual(["read", "glob", "grep", "list", "bash", "job_output", "job_list", "job_stop", "web_search", "web_fetch", "report"])
     // the preamble is the spec's text; a subagent is told nothing of plan_write or ask_user