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/common/kigi-interjection-core/src/format.rs

@@ -1,4 +1,4 @@
-/// Truncation threshold, matching the shell's large-prompt limit.
+/// Truncation threshold; matches the shell's large-prompt limit.
 pub const LARGE_PROMPT_THRESHOLD: usize = 25_000;
 
 /// Wrap a user message in the canonical `<user_query>` envelope.
@@ -10,9 +10,9 @@ pub fn user_query(user_message: &str) -> String {
     )
 }
 
-/// Wrap interjection text as a synthetic user message with a mid-turn note.
-/// No deferral instruction: the model decides how to weigh it against
-/// in-flight work. Output is byte-identical to the shell's historical format.
+/// Frame interjection text as a synthetic mid-turn user message.
+/// No deferral instruction — the model weighs it against in-flight work.
+/// Byte-identical to the shell's historical format.
 pub fn format_interjection(text: String) -> String {
     let truncated = if text.len() > LARGE_PROMPT_THRESHOLD {
         let end = text