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/config_model_override_parse.rs

@@ -17,7 +17,6 @@ use serde::Serialize;
 
 use super::config::ConfigModelOverride;
 
-/// Category for a [`ModelOverrideWarning`].
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize)]
 #[serde(rename_all = "kebab-case")]
 pub enum ModelOverrideWarningKind {
@@ -48,7 +47,6 @@ pub struct ModelOverrideWarning {
     pub reason: String,
 }
 
-/// Result of [`parse_model_overrides`].
 pub(crate) struct ParsedModelOverrides {
     pub models: IndexMap<String, ConfigModelOverride>,
     pub warnings: Vec<ModelOverrideWarning>,