brokerd: runbook pointers for startup failures, no thread panics, args_os

M3a review findings 3, 5 (the cast), 6, 7 (brokerd), 11. A config, directory
or socket failure at start now ends with docs/runbook.md#brokerd-start-failed,
and losing a listener with #brokerd-listener-lost; both entries are new.
Threads start through thread::Builder, so a refused thread is reported
instead of silently killing a listener; an aborted connection no longer stops
the daemon. brokerd reads args_os and keeps the config path as a path. The
"requester went away" result is recorded at the time it happens.

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

crates/brokerd/src/args.rs

@@ -6,7 +6,7 @@
 
 use serde::{Deserialize, Serialize};
 use std::fmt;
-
+/// Maximum length, in bytes, of a path.
 /// Maximum length, in bytes, of a path or of a URL.
 pub const MAX_PATH: usize = 4096;
 /// Maximum length, in bytes, of a URL.