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/agent/subagent/handle_request.rs

@@ -36,11 +36,6 @@ pub(super) fn task_model_override_error(
         is_session_auth,
     )
 }
-/// This is a free async function, NOT a method on MvpAgent. It receives
-/// a `SubagentSpawnContext` with everything it needs, and a mutable
-/// reference to the coordinator for tracking.
-///
-/// Returns when the child session completes (or fails/cancels).
 #[tracing::instrument(
     name = "subagent.handle_request",
     skip_all,