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-fast-worktree/src/discovery.rs

@@ -299,7 +299,6 @@ mod tests {
         assert!(db.get(&wt_a.to_string_lossy()).unwrap().is_some());
         assert!(db.get(&wt_b.to_string_lossy()).unwrap().is_some());
 
-        // Idempotent: a second rebuild finds both already tracked, skips neither.
         let report2 = rebuild_worktree_db(&db, kigi_home).unwrap();
         assert_eq!(report2.registered, 0);
         assert_eq!(report2.already_tracked, 2);