Add M2b plan: ten tasks, tests, recordings and the first system prompt

The tasks build the agent loop on M2a's client: channel messages and
the usage record in proto, four config tables, the tool port and
registry with find_tool and call_tool, the baseline and replay, the
session store, the turn loop with its limits and the append-only
property test, the channel server, loopd serve, bxctl chat, and the
device checks including a four-turn conversation with a restart.

Checked against a private reference implementation: the gate passes
after every task in order, the new suites pass under CPU load, and the
reference passes make verify-device on straylight with no cache loss.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

CLAUDE.md

@@ -7,7 +7,8 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 Boxmaker is a sovereign personal agent harness written in Rust. Work proceeds one milestone at a
 time (M0 to M7, table in `docs/milestones.md`). M0 (measurements) and M1 (workspace, `proto`, gate,
 `Decision`) and M2a (the inference path: `inferproxy`, `loopd`'s HTTP, SSE and llama client,
-the startup self-test) are done. M2b (sessions, turn loop, `bxctl chat`) is not designed yet. Check `docs/implementer-log.md` for
+the startup self-test) are done. M2b (sessions, turn loop, `bxctl chat`) is specified in
+`docs/specs/2026-09-18-m2b-agent-loop.md` and planned in `docs/plans/M2b/`. Check `docs/implementer-log.md` for
 what is in flight.
 
 - `docs/design.md` is the binding design brief. If it looks wrong or conflicts with a measurement,