docs(comments): rewrite comments across all crates to the guidelines

Sweep every first-party crate source (1956 .rs files) to the project comment
guidelines: delete redundant restatements, decorative banners, change
narration, and end-of-line comments; keep and tighten the crucial ones
(invariants, bug rationale, SAFETY blocks, ported-source attribution).

No functional code changed. Every edit is proven comment-only against the
prior tree by a comment-stripping lexer (string/char/raw-string aware) plus a
separate doctest-fence check. Where removing a comment made rustfmt or clippy
want to re-lay-out adjacent code, the minimal triggering comment is restored so
code tokens stay byte-identical.

Gates green: cargo fmt --all --check (0 diffs), cargo check and cargo clippy
--workspace --all-targets (0 warnings).

Adds scripts/check_codegen_comment_guidelines.py — the enforcement gate for
these guidelines (flags banners, end-of-line comments, change narration, and
commented-out code).

crates/codegen/kigi-acp-lib/src/channel.rs

@@ -78,7 +78,8 @@ mod acp_send_failure_tests {
     #[tokio::test]
     async fn send_failed_when_receiver_dropped_before_send() {
         let (tx, rx) = mpsc::unbounded_channel::<AcpAgentMessage>();
-        drop(rx); // no peer listening -> enqueue fails
+        // no peer listening -> enqueue fails
+        drop(rx);
         let err = acp_send(ext_request(), &tx).await.unwrap_err();
         assert_eq!(
             acp_channel_failure(&err),