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-pager-pty-harness/src/host_clipboard.rs

@@ -19,7 +19,6 @@ use std::process::{Command, Stdio};
 
 use anyhow::{Context, Result, bail};
 
-/// Copy `text` to the host clipboard via `pbcopy`.
 #[cfg(not(target_os = "windows"))]
 pub fn pbcopy(text: &str) -> Result<()> {
     let mut cmd = Command::new("pbcopy");
@@ -152,7 +151,6 @@ pub fn set_clipboard_png(path: &Path) -> Result<()> {
     Ok(())
 }
 
-/// Write a small solid-color PNG under `dir` and return its path.
 pub fn write_fixture_png(dir: &Path) -> Result<PathBuf> {
     let path = dir.join("host_clipboard_fixture.png");
     let buf: image::ImageBuffer<image::Rgba<u8>, Vec<u8>> =