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>

docs/specs/2026-09-18-m2b-agent-loop.md

@@ -142,11 +142,13 @@ Events for one turn arrive in order on one connection. Nothing else is promised.
 
 `bxctl chat [--socket <path>] [--session <id>] [--no-thinking] [--say <text>] [--json]`
 
+- A `--session` id that does not exist yet is created (the first turn is retried with
+  `resume: false` when `loopd` answers `no_such_session`), so scripts can choose their ids.
 - Prints the session id, then reads one line at a time from stdin and runs a turn per line.
   Content is printed as it streams; reasoning is printed dimmed; each tool call is one line;
   `waiting`, `retrying` and `thinking_capped` are one status line each. EOF or `/quit` ends it.
 - `--say` runs one turn and exits with the answer on stdout and status 0, or the error on stderr
-  and status 1.
+  and status 1. Its events go to stderr, so stdout carries only the answer.
 - `--json` prints every frame body as one JSON line, for scripts.
 - No readline, no history. The socket path defaults to `<BOXMAKER_HOME>/run/loop/loop.sock`.
 
@@ -155,7 +157,10 @@ Events for one turn arrive in order on one connection. Nothing else is promised.
 `loopd serve --config <path>`: runs the self-test and exits 1 if it fails; binds `loop.sock`
 (from config, `[channel] socket`) with mode 0600; accepts connections, one thread each. The M2a
 `Client` is shared in an `Arc`; its slot gate serialises requests per slot. A registry of
-per-session locks makes a concurrent `turn` on a busy session answer `session_busy` at once.
+per-session locks makes a concurrent `turn` on a busy session answer `session_busy` at once; the
+lock is released before the final frame is sent, so a channel that sends its next turn on reading
+that frame is never refused. A stale socket file is removed before the self-test runs, so clients
+see "not running" rather than "connection refused" during startup.
 Nothing is held in memory that is not also on disk, except the locks and the `call` counters.
 
 Config additions: `[channel] socket`, `[paths] home` (default `BOXMAKER_HOME` or