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/common/kigi-tool-runtime/tests/tool_streaming.rs

@@ -150,7 +150,6 @@ async fn streaming_err_propagates_through_terminal() {
 
 #[tokio::test]
 async fn streaming_progress_count_is_independent_of_args() {
-    // Distinct invocations on the same tool produce the same shape.
     let tool = StreamingOk;
     for _ in 0..3 {
         let count = tool
@@ -164,8 +163,7 @@ async fn streaming_progress_count_is_independent_of_args() {
 
 #[tokio::test]
 async fn empty_progress_still_yields_terminal() {
-    // Building `with_progress` on an empty stream still produces exactly
-    // one terminal item — the same shape `terminal_only` produces.
+    // Empty progress stream still yields exactly one terminal item.
     let progress = stream::iter(Vec::<ToolProgress>::new());
     let mut stream = with_progress(progress, async move { Ok::<u32, ToolError>(99) });
     let item = stream.next().await.unwrap();