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/active_sessions.rs

@@ -23,8 +23,6 @@ const DATA_FILENAME: &str = "active_sessions.json";
 const LOCK_FILENAME: &str = "active_sessions.lock";
 const TMP_FILENAME: &str = "active_sessions.json.tmp";
 
-// -- Public API (delegates to `_in` variants with default kigi home) --------
-
 /// Register a session as active (idempotent by session_id).
 pub fn register(session: ActiveSession) -> io::Result<()> {
     register_in(&crate::util::kigi_home::kigi_home(), session)
@@ -46,8 +44,6 @@ pub fn collect_crashed() -> io::Result<Vec<ActiveSession>> {
     collect_crashed_in(&crate::util::kigi_home::kigi_home())
 }
 
-// -- Injectable-root variants (`_in`) for testing ---------------------------
-
 pub fn register_in(root: &Path, session: ActiveSession) -> io::Result<()> {
     with_locked_state(root, |sessions| {
         sessions.retain(|s| s.session_id != session.session_id);
@@ -81,8 +77,6 @@ pub fn list_in(root: &Path) -> io::Result<Vec<ActiveSession>> {
     read_data_file(&data_path)
 }
 
-// -- Internal: locked read-modify-write -------------------------------------
-
 fn with_locked_state<F, R>(root: &Path, mutate: F) -> io::Result<R>
 where
     F: FnOnce(&mut Vec<ActiveSession>) -> R,