Specify and plan M3b: the runner and the tools

A draft spec for the owner's review and 13 offline tasks with their given
tests: shared tool arguments and host rules in proto, the sealed fetch
target (M3a finding 14), the toolkit tools and SOCKS5 egress proxy, and
brokerd's [runner], podman argument lists, runtime and proxy lifecycle. Each
task's tests were run against a reference at that task's end state (560 to
638 tests, clippy clean); the reference is not in the repository. Adds the
runner-unavailable runbook entry and tip T23 (ETXTBSY in script tests).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/plans/M3b/09-brokerd-runner-config.md

@@ -0,0 +1,100 @@
+# M3b task 09: `[runner]` in `brokerd.toml`
+
+**Branch:** `m3b` (run `git switch m3b`; `git status --short` must be empty, otherwise stop)
+**Commit subject:** `brokerd: the [runner] section`
+
+## Goal
+
+`[runner]` says how tools run: which `podman`, which image, and the limits. Without the section,
+`brokerd` keeps M3a's runtime, which refuses every call (task 13 does that choice). The image must
+be named **by digest**, so what runs is exactly what was built. Spec section 6, "Configuration".
+
+## Files
+
+- Copy: `crates/brokerd/tests/config_runner.rs`, the five files
+  `crates/brokerd/tests/fixtures/config/runner_*.toml`, and `crates/brokerd/tests/support/rig.rs`
+  (replaces the old one: it builds a `Config` with a struct literal, which now needs
+  `runner: None`)
+- Modify: `crates/brokerd/src/config.rs`, `docs/implementer-log.md`
+
+## Interfaces
+
+In `config.rs`:
+
+```rust
+#[derive(Debug, Clone, PartialEq, Eq, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct Runner {
+    #[serde(default = "default_podman")]         pub podman: PathBuf,        // "podman"
+    pub image: String,                                                       // required, no default
+    #[serde(default = "default_egress_network")] pub egress_network: String, // "pasta"
+    #[serde(default = "default_output_cap")]     pub output_cap: u64,        // 262_144
+    #[serde(default = "default_memory")]         pub memory: String,         // "512m"
+    #[serde(default = "default_pids")]           pub pids: u32,              // 128
+    #[serde(default = "default_read_file_ms")]   pub read_file_ms: u64,      // 30_000
+    #[serde(default = "default_write_file_ms")]  pub write_file_ms: u64,     // 30_000
+    #[serde(default = "default_shell_ms")]       pub shell_ms: u64,          // 100_000
+    #[serde(default = "default_http_fetch_ms")]  pub http_fetch_ms: u64,     // 60_000
+}
+
+impl Runner {
+    /// The time limit for one call of `tool`: the matching `_ms` field.
+    pub fn time_limit(&self, tool: crate::args::ToolName) -> std::time::Duration;
+}
+
+pub struct Config {
+    // paths, sockets, approvals as now, then:
+    #[serde(default)]
+    pub runner: Option<Runner>,
+}
+
+impl Config {
+    // … as now, plus:
+    /// `<home>/run/egress`: where each `http_fetch` call gets a directory for the proxy's socket.
+    pub fn egress_dir(&self) -> PathBuf;
+}
+```
+
+Write one small private `fn default_…() -> …` per defaulted field (serde needs a function path).
+Put the struct attributes on separate lines as usual; the table above only saves space.
+
+## Checks in `Config::load`
+
+After the `ttl_ms` check, if `runner` is `Some`, check it and return the first problem as
+`ConfigError::Invalid(path, text)`. In this order:
+
+1. `image` is not `<name>@sha256:<hex>` where `<name>` is not empty and `<hex>` is exactly 64 of
+   `0-9a-f` (lowercase). Split with `rsplit_once("@sha256:")`. Text:
+   `[runner] image is {image:?}; it must be named by digest: <name>@sha256:<64 hex digits>`.
+2. `memory` is not digits followed by one of `b`, `k`, `m`, `g` (at least one digit). Text:
+   `[runner] memory is {memory:?}; it must be a number and one of b, k, m, g`.
+3. `egress_network` or `podman` is empty. Text:
+   `[runner] podman and egress_network must not be empty`.
+4. Any of `output_cap`, `pids`, `read_file_ms`, `write_file_ms`, `shell_ms`, `http_fetch_ms` is 0,
+   checked in that order. Text: `[runner] <name> must be at least 1`, with the field's name.
+
+A missing `image` or an unknown key is already a `ConfigError::Parse` from serde; do not add
+checks for those. `Config::parse` (the string version the tests use for plain values) does not run
+these checks; only `load` does, as for `ttl_ms` today.
+
+## Steps
+
+- [ ] **1. Copy.** `git switch m3b`, then
+  `cp docs/plans/M3b/files/crates/brokerd/tests/config_runner.rs crates/brokerd/tests/`,
+  `cp docs/plans/M3b/files/crates/brokerd/tests/fixtures/config/runner_*.toml crates/brokerd/tests/fixtures/config/`,
+  `cp docs/plans/M3b/files/crates/brokerd/tests/support/rig.rs crates/brokerd/tests/support/`
+- [ ] **2. See it fail.** `cargo test -p brokerd --test config_runner`. Expected: it does not
+  compile.
+- [ ] **3. Write the code.** Run `cargo fmt --all`.
+- [ ] **4. See it pass.** `cargo test -p brokerd --test config_runner --test config`. Expected: 7
+  and 7 passed. Then `cargo test -p brokerd`: everything passes.
+- [ ] **5. Run the gate.** `make gate`. Expected last line: `gate: ok`.
+- [ ] **6. Log and commit.** `git add crates/brokerd docs/implementer-log.md && git commit`
+
+## Done when
+
+- `cargo test -p brokerd` passes; `make gate` prints `gate: ok`.
+
+## Stop and report if
+
+- A test wants an image named by tag (`:latest`) to be accepted.