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-shell/src/sampling/error.rs

@@ -1,10 +1,9 @@
 //! Sampling error types.
 //!
-//! The canonical error types now live in `kigi_sampling_types::error`.
-//! This module re-exports them and adds `map_sampling_err_to_acp` which
-//! depends on `agent_client_protocol::Error` (a kigi-shell dependency).
+//! The canonical error types live in `kigi_sampling_types::error`. This module
+//! re-exports them and adds `map_sampling_err_to_acp`, which depends on
+//! `agent_client_protocol::Error` (a kigi-shell dependency).
 
-// Re-export everything from the standalone crate.
 pub use kigi_sampling_types::error::*;
 
 use agent_client_protocol as acp;
@@ -51,7 +50,6 @@ pub fn rate_limited_user_message(is_api_key_auth: bool) -> &'static str {
 }
 
 /// Map a `SamplingError` to an ACP `Error` for client-facing responses.
-/// This stays in kigi-shell because it depends on `agent_client_protocol::Error`.
 pub fn map_sampling_err_to_acp(err: SamplingError) -> acp::Error {
     use reqwest::StatusCode;
     match err {
@@ -339,7 +337,6 @@ mod tests {
         assert!(RATE_LIMITED_USER_MESSAGE_OAUTH.contains("official Kimi CLI"));
         assert!(RATE_LIMITED_USER_MESSAGE_OAUTH.contains("same subscription quota"));
         assert!(RATE_LIMITED_USER_MESSAGE_OAUTH.contains(kigi_env::upgrade_page_url()));
-        // API-key copy points at the Moonshot platform, not the subscription.
         assert!(RATE_LIMITED_USER_MESSAGE_API_KEY.contains("Moonshot"));
         assert!(!RATE_LIMITED_USER_MESSAGE_API_KEY.contains("subscription quota"));
     }
@@ -459,7 +456,6 @@ mod tests {
             }
         }
         let result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(f));
-        // Restore original state.
         unsafe {
             std::env::remove_var("KIGI_API_KEY");
             std::env::remove_var("XAI_API_KEY");