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-log/src/hooks_log.rs

@@ -112,7 +112,8 @@ fn resolve_log_path() -> Option<PathBuf> {
     let default_path = || kigi_home().join("logs").join("hooks.log");
     let raw = match std::env::var(ENV_HOOKS_LOG) {
         Ok(val) => val,
-        Err(_) => return None, // opt-in only
+        // opt-in only
+        Err(_) => return None,
     };
     let raw = raw.trim();
     match raw {