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-render/src/terminal/embedded_editor.rs

@@ -31,7 +31,7 @@ pub enum EmbeddedEditor {
 /// [`EmbeddedEditor::Emacs`]; else `None`. Empty values are treated as absent
 /// (matching the sibling `detect_*_from_env` detectors via `env_get`).
 ///
-/// Adding a new env marker here requires extending
+/// Including a new env marker here requires extending
 /// `HOST_TERMINAL_ENV_VARS` in `kigi-pager-pty-harness/src/pty.rs`
 /// (test-env hygiene).
 pub fn embedded_editor_from_env(env: &HashMap<String, String>) -> Option<EmbeddedEditor> {