# 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#`.)