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-tools/src/util/env.rs

@@ -1,9 +1,8 @@
 //! Environment variable helpers and process isolation for terminal execution.
 //!
-//! All implementations now live in the lightweight [`kigi_tty_utils`] crate
-//! so that every crate in the workspace can use them without pulling in the
-//! heavyweight `kigi-tools` dependency. This module re-exports the public
-//! API for backward compatibility.
+//! The implementations live in the lightweight [`kigi_tty_utils`] crate so
+//! every workspace crate can use them without depending on `kigi-tools`; this
+//! module re-exports the public API.
 
 pub use kigi_tty_utils::{detach_from_tty, pager_env};