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/agent/feedback_client.rs

@@ -24,7 +24,6 @@ pub struct FeedbackApiError {
 }
 
 impl FeedbackApiError {
-    /// Returns `true` if this is a 401 Unauthorized response.
     pub fn is_unauthorized(&self) -> bool {
         self.status == reqwest::StatusCode::UNAUTHORIZED
     }
@@ -49,7 +48,6 @@ enum BearerSource {
     Static(String),
 }
 
-/// Client for the Kimi Code feedback endpoint.
 #[derive(Clone)]
 pub struct FeedbackClient {
     http: reqwest::Client,
@@ -73,7 +71,6 @@ impl FeedbackClient {
         }
     }
 
-    /// Client with a fixed Bearer token (tests).
     #[cfg(test)]
     pub(crate) fn with_static_token(base_url: impl Into<String>, token: impl Into<String>) -> Self {
         Self {