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/graph_planner.rs

@@ -520,7 +520,8 @@ mod tests {
     async fn oversize_artifact_is_invalid_with_cap_in_reason() {
         let (_tmp, target) = tmp_graph_file("oversize");
         let mut body = vec![b'x'; (MAX_GRAPH_JSON_BYTES as usize) + 1];
-        body[0] = b'{'; // content is irrelevant; the size gate fires first
+        // Content is irrelevant; the size gate fires first.
+        body[0] = b'{';
         let spawner = MockSpawner {
             response: MockReply::Done,
             body: Some(body),