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/08-loopd-serve.md

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+# M2b task 08: `loopd serve`
+
+**Branch:** `m2b` (run `git switch m2b`; `git status --short` must be empty, otherwise stop)
+**Commit subject:** `Add the loopd serve command`
+
+## Goal
+
+The command that runs the harness: self-test, then the channel server. It refuses to start if the
+self-test fails.
+
+## Files
+
+- Copy: `crates/loopd/tests/serve.rs`
+- Modify: `crates/loopd/src/main.rs`, `docs/implementer-log.md`
+
+## What `loopd serve --config <path>` does, in order
+
+1. Load the config; on failure print `loopd: <error>` and exit 1.
+2. Work out the socket path with `cfg.channel_socket()`. If a socket file is already there,
+   remove it **now**, before the self-test, so that a client sees "not running" rather than
+   "connection refused" while the self-test runs.
+3. Run the self-test exactly as `loopd selftest` does, printing the same lines. On failure exit 1
+   without binding anything.
+4. Create the socket's parent directory, bind, set the socket file's mode to 0600, print
+   `loopd: serving on <path>`, and call `channel::serve` with a `Context` made of the config, the
+   client, `Box::new(FakeTools::new())` and `Registry::m2b()`.
+
+`loopd selftest --config <path>` keeps working as before. Anything else prints the usage for both
+commands and exits 2.
+
+## Steps
+
+- [ ] **1. Copy.** `git switch m2b`, then
+  `cp docs/plans/M2b/files/crates/loopd/tests/serve.rs crates/loopd/tests/`
+- [ ] **2. See the test fail.** `cargo test -p loopd --test serve`. Expected: all 3 fail, because
+  `serve` is not a command yet.
+- [ ] **3. Rewrite `main.rs`.** Keep the `selftest` command. Run `cargo fmt --all`.
+- [ ] **4. See the tests pass.** `cargo test -p loopd --test serve`. Expected: `3 passed`.
+- [ ] **5. Run the gate.** `make gate`. Expected last line: `gate: ok`.
+- [ ] **6. Log and commit.** `git add crates/loopd docs/implementer-log.md && git commit`
+
+## Done when
+
+- `cargo test -p loopd --test serve` reports 3 passed; `make gate` prints `gate: ok`.
+
+## Stop and report if
+
+- The socket cannot be given mode 0600 with `std::fs::set_permissions`.