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/02-loopd-config.md

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+# M2b task 02: the M2b config tables
+
+**Branch:** `m2b` (run `git switch m2b`; `git status --short` must be empty, otherwise stop)
+**Commit subject:** `Add the paths, channel, loop and baseline config tables`
+
+## Goal
+
+Four small config tables that the rest of M2b reads. All are optional, with defaults, and reject
+unknown keys like the existing ones.
+
+```toml
+[paths]
+home = "/var/lib/boxmaker"          # default: $BOXMAKER_HOME if set, else this
+
+[channel]
+socket = ""                         # default empty, meaning <home>/run/loop/loop.sock
+
+[loop]
+tool_iterations = 8                 # completions with tool calls allowed in one turn
+repeat_detection = true             # stop the same call made twice in a turn
+tool_result_cap = 16384             # bytes of a tool result that are kept
+
+[baseline]
+system = "system.md"                # the system prompt; relative to the config file's directory
+```
+
+## Files
+
+- Copy (replacing the old one): `crates/loopd/tests/config.rs`;
+  new: `crates/loopd/tests/fixtures/config/m2b.toml`
+- Modify: `crates/loopd/src/config.rs`, `docs/implementer-log.md`
+
+## Interfaces
+
+Added to `Config` (all `pub`, all `#[serde(default)]`): `paths: Paths`, `channel: Channel`,
+`r#loop: Loop`, `baseline: Baseline`. The field is spelled `r#loop` because `loop` is a keyword;
+in TOML it is `[loop]`.
+
+```rust
+pub struct Paths { pub home: PathBuf }              // Default: $BOXMAKER_HOME or /var/lib/boxmaker
+pub struct Channel { pub socket: PathBuf }          // Default: empty
+pub struct Loop { pub tool_iterations: u32, pub repeat_detection: bool, pub tool_result_cap: usize }
+pub struct Baseline { pub system: PathBuf }         // Default: "system.md"
+
+impl Config {
+    /// The channel socket, with the default filled in.
+    pub fn channel_socket(&self) -> PathBuf;
+}
+```
+
+Derives: `Debug, Clone, PartialEq, Eq, Deserialize` and `#[serde(deny_unknown_fields, default)]`
+on all four, each with a hand-written `Default` (or derived, for `Channel`).
+
+Rule: `Config::load` makes a relative `baseline.system` absolute by joining it to the config
+file's directory. `Config::parse` leaves it as it is, because it has no file to be relative to.
+
+## Steps
+
+- [ ] **1. Copy.**
+
+```sh
+git switch m2b
+cp docs/plans/M2b/files/crates/loopd/tests/config.rs crates/loopd/tests/
+cp docs/plans/M2b/files/crates/loopd/tests/fixtures/config/m2b.toml crates/loopd/tests/fixtures/config/
+```
+
+- [ ] **2. See the tests fail.** `cargo test -p loopd --test config`. Expected: it does not compile.
+- [ ] **3. Make the changes.** Run `cargo fmt --all`.
+- [ ] **4. See the tests pass.** `cargo test -p loopd --test config`. Expected: `9 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 config` reports 9 passed; `make gate` prints `gate: ok`.
+- `grep -c deny_unknown_fields crates/loopd/src/config.rs` prints 10 (the six old structs and
+  the four new ones).
+
+## Stop and report if
+
+- `r#loop` cannot be used as a field name with serde. (It can; `serde` strips the `r#`.)