docs(#2552): never add #[allow(clippy::...)] — fix lints instead

add a clippy-discipline note to docs/conventions.md under 'Building &
local checks' and a short pointer bullet in CLAUDE.md so the rule is
visible at first read. covers the three most common patterns that surfaced
in practice (too_many_lines → extract helper, doc_markdown → backticks,
must_use → handle or discard) and gives a concrete worked example
(TurnAccum extraction in stats.rs).

CLAUDE.md

@@ -165,6 +165,9 @@ The docs below own the details — this section just points at them.
 
 - **Commit style, naming, identity, reconcile verb:** →
   [`docs/conventions.md`](docs/conventions.md).
+- **Never add `#[allow(clippy::…)]`** — fix the lint instead (extract a
+  helper, add backticks, etc.). Details + worked examples: →
+  [`docs/conventions.md`](docs/conventions.md#building--local-checks).
 - **NixOS / nspawn quirks** (bind mounts, conf flags, etc.): →
   [`docs/gotchas.md`](docs/gotchas.md).
 - **Turn loop, sentinels (rate-limit, auth-failed), context