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/session/storage/jsonl/mod.rs

@@ -30,9 +30,7 @@ async fn replace_file_async(tmp: PathBuf, target: PathBuf) -> io::Result<()> {
 ///   Used for subagent child sessions whose files live under the parent's session dir.
 #[derive(Clone)]
 enum SessionDirMode {
-    /// Existing behavior: root + sessions/{cwd}/{id}/
     FromRoot(PathBuf),
-    /// New: use this directory directly (for subagent children).
     Explicit(PathBuf),
 }
 /// JSONL-based storage adapter (legacy format)
@@ -319,7 +317,6 @@ impl JsonlStorageAdapter {
         }
         Ok(items)
     }
-    /// Append a session update to the updates.jsonl file, wrapping it in an envelope with timestamp.
     async fn append_update_to_file(
         &self,
         path: PathBuf,
@@ -605,7 +602,6 @@ impl JsonlStorageAdapter {
         .map_err(io::Error::other)?
     }
 }
-/// Transform session ID in a SessionUpdate
 fn transform_session_id_in_update(
     update: super::SessionUpdate,
     new_id: &acp::SessionId,