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/src/trace_cmd.rs

@@ -30,10 +30,6 @@ pub async fn run(args: TraceArgs, _agent_config: &AgentConfig) -> Result<()> {
     run_export(&args.session_id, args.output.as_deref(), args.json).await
 }
 
-// ---------------------------------------------------------------------------
-// Archive construction
-// ---------------------------------------------------------------------------
-
 pub fn build_session_tar(session_dir: &Path, session_id: &str) -> Result<Vec<u8>> {
     use flate2::Compression;
     use flate2::write::GzEncoder;
@@ -151,10 +147,6 @@ fn add_directory_to_tar<W: std::io::Write>(
     Ok(count)
 }
 
-// ---------------------------------------------------------------------------
-// Local export
-// ---------------------------------------------------------------------------
-
 pub(crate) fn find_session_dir(session_id: &str) -> Result<PathBuf> {
     kigi_shell::session::persistence::find_session_dir_by_id(session_id).with_context(|| {
         format!(