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-chat-state/src/lib.rs

@@ -1,8 +1,7 @@
 //! kigi-chat-state — Actor-based chat state management for xAI agents.
 //!
-//! This crate extracts conversation state management from `kigi-shell`'s
-//! `acp_session.rs` into a standalone actor. It follows the same actor pattern
-//! as `kigi-hunk-tracker`:
+//! Holds the conversation state driven by `kigi-shell`'s `acp_session.rs`,
+//! following the same actor pattern as `kigi-hunk-tracker`:
 //!
 //! ```text
 //! ┌────────────────┐                  ┌──────────────────────────────────────┐
@@ -35,7 +34,6 @@ pub mod persistence;
 pub mod types;
 pub mod usage;
 
-// Re-export main types for convenience
 pub use actor::ChatStateActor;
 pub use actor::state::{
     estimate_conversation_tokens, estimate_item_tokens, estimate_messages_tokens,