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/Makefile

@@ -0,0 +1,29 @@
+# Boxmaker gate. `make gate` must pass before any work is called done. It needs no network.
+
+.PHONY: gate audit verify-device
+
+gate:
+	cargo fmt --all --check
+	cargo clippy --workspace --all-targets --locked --offline -- -D warnings
+	cargo test --workspace --locked --offline
+	cargo deny --offline check bans licenses sources
+	sh scripts/check-lines.sh
+	sh scripts/check-crate-deps.sh
+	sh scripts/check-dep-docs.sh
+	sh scripts/test-gate-scripts.sh
+	@echo "gate: ok"
+
+# Fetches the RustSec advisory database. Listed in docs/egress.md.
+audit:
+	cargo deny check advisories
+
+# Checks that need straylight. They go through a private inferproxy to the real server.
+# Override the server with: make verify-device UPSTREAM=host:port
+UPSTREAM ?= straylight:11434
+
+verify-device:
+	cargo build --workspace --locked
+	BOXMAKER_INFERPROXY=$(CURDIR)/target/debug/inferproxy BOXMAKER_BXCTL=$(CURDIR)/target/debug/bxctl \
+		BOXMAKER_UPSTREAM=$(UPSTREAM) \
+		cargo test -p loopd --test device --locked -- --ignored --test-threads=1
+	@echo "verify-device: ok"