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-config/src/managed_cache.rs

@@ -425,7 +425,8 @@ fn managed_config_stale_at(home: Option<&Path>, identity: &ServingIdentity) -> b
         return false;
     };
     let Some(cache) = read_managed_config_cache(home) else {
-        return true; // no marker → never synced → stale
+        // no marker → never synced → stale
+        return true;
     };
     if cache_unusable_for(&cache, home, identity) {
         return true;