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-tui/tests/pty_auto_mode.rs

@@ -149,7 +149,8 @@ fn pty_shift_tab_cycles_to_auto_mode_banner() {
             harness.is_running() || !early.is_empty(),
             "pager must have produced output even on login screen"
         );
-        let _ = harness.inject_keys(b"\x11"); // ctrl+q if bound
+        // ctrl+q if bound
+        let _ = harness.inject_keys(b"\x11");
         return;
     }