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/10-verify-device.md

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+# M2b task 10: verification on the real server
+
+**Branch:** `m2b` (run `git switch m2b`; `git status --short` must be empty, otherwise stop)
+**Commit subject:** `Extend on-device verification to the agent loop`
+
+## Goal
+
+Run the whole loop against straylight: the token budget of the real baseline, and a four-turn
+conversation through `loopd serve` and `bxctl chat` with a `loopd` restart in the middle. You
+write no library code. **If a check fails, that is a finding. Do not change a test or a limit.**
+
+## Files
+
+- Copy: `crates/loopd/tests/device.rs` (replaces the M2a one; the four M2a checks are still in
+  it), `Makefile` (only `verify-device` changes: it now also passes `BOXMAKER_BXCTL`), and
+  `config/system.md` (the first system prompt; the device test reads it from the repository)
+- Modify: `docs/implementer-log.md`
+
+## Steps
+
+- [ ] **1. Copy.**
+
+```sh
+git switch m2b
+cp docs/plans/M2b/files/crates/loopd/tests/device.rs crates/loopd/tests/
+cp docs/plans/M2b/files/Makefile Makefile
+mkdir -p config && cp docs/plans/M2b/files/config/system.md config/
+git diff --stat Makefile
+```
+
+- [ ] **2. The ordinary gate ignores the new tests.** `make gate`. Expected last line: `gate: ok`,
+  and for `device`: `0 passed; 0 failed; 6 ignored`.
+- [ ] **3. The server can be reached.** `curl -s http://straylight:11434/health`. Expected:
+  `{"status":"ok"}`. If not, stop and report.
+- [ ] **4. Run the device checks.** `make verify-device`. Expected: `6 passed` and
+  `verify-device: ok`, in about a minute. The baseline token count is printed; write it in your
+  log row.
+- [ ] **5. If a check failed**, run it alone with `--nocapture` (see the M2a task 13 for the
+  command) and put its name and message in your log row with status `stopped`.
+- [ ] **6. Log and commit.**
+
+```sh
+git add Makefile config crates/loopd/tests/device.rs docs/implementer-log.md
+git commit
+```
+
+## Done when
+
+- `make gate` prints `gate: ok` and `make verify-device` prints `verify-device: ok`.
+- `cmp Makefile docs/plans/M2b/files/Makefile` and `cmp config/system.md docs/plans/M2b/files/config/system.md` print nothing.
+- `git log --oneline master..m2b` shows one commit per task.
+
+## Stop and report if
+
+- The server cannot be reached, or any device check fails twice.