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/common/kigi-tool-runtime/tests/search.rs

@@ -93,10 +93,8 @@ fn tool_index_wrapper_clones_arc() {
     });
     let wrapped = ToolIndex(inner.clone());
     let copy = wrapped.clone();
-    // Both wrappers hold the same Arc — strong-count includes both
-    // wrappers and the original `inner` binding.
+    // Both wrappers share the Arc with `inner`.
     assert!(Arc::strong_count(&inner) >= 3);
-    // Debug impl renders without leaking the inner type.
     let debug = format!("{wrapped:?}");
     assert_eq!(debug, "ToolIndex");
     drop(copy);