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/session/goal_next_step.rs

@@ -91,7 +91,7 @@ fn first_unchecked_in_checklist(body: &str) -> Option<String> {
             continue;
         };
         if is_any_header(line) && header_level(line) <= level {
-            return None; // section ended; no unchecked item found
+            return None;
         }
         if let Some(item) = parse_checkbox_item(line.trim_start()) {
             return Some(item);
@@ -126,8 +126,6 @@ fn extract_first_unchecked(body: &str) -> Option<String> {
     None
 }
 
-/// Strip the leading `- ` / `* ` / `+ ` bullet marker. `None` when
-/// the line does not begin with a recognised bullet glyph.
 fn strip_bullet_marker(trimmed: &str) -> Option<&str> {
     trimmed
         .strip_prefix("- ")