Add brokerd's configuration

Implemented crates/brokerd/src/config.rs: typed Paths, Sockets, Approvals and
Config with serde(deny_unknown_fields, default) on every struct, hand-written
ConfigError (Read/Parse) with Display and std::error::Error, and the parse/load/
broker_socket/admin_socket/audit_dir/state_dir methods. Added serde, serde_json
and toml to crates/brokerd/Cargo.toml, registered pub mod config; in lib.rs,
added brokerd to the serde and serde_json Used-by cells in docs/dependencies.md,
and copied the given test and six fixtures byte-identical. 7 config tests pass;
make gate prints gate: ok.

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/brokerd/tests/fixtures/config/empty.toml

@@ -0,0 +1 @@
+# Nothing set: every value is a default.