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/plans/M2b/files/crates/loopd/tests/fixtures/config/m2b.toml

@@ -0,0 +1,26 @@
+[infer]
+socket = "/tmp/infer.sock"
+model = "some-model"
+
+[slots]
+main = 0
+background = 1
+
+[expect]
+template_sha256 = "000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f"
+n_ctx = 4096
+slots = 2
+
+[paths]
+home = "/srv/boxmaker"
+
+[channel]
+socket = "/run/boxmaker/loop/loop.sock"
+
+[loop]
+tool_iterations = 3
+repeat_detection = false
+tool_result_cap = 1024
+
+[baseline]
+system = "prompts/agent.md"