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_blocking.rs

@@ -151,8 +151,7 @@ async fn unimplemented_tool_returns_not_implemented_terminal() {
 
 #[tokio::test]
 async fn run_takes_args_by_value() {
-    // The trait `run` consumes args; this would not compile if the
-    // signature accidentally borrowed.
+    // run consumes args (would not compile if the signature borrowed).
     let tool = BlockingOk;
     let args = EchoArgs {
         text: "consumed".into(),
@@ -163,7 +162,6 @@ async fn run_takes_args_by_value() {
 
 #[tokio::test]
 async fn execute_default_drains_in_one_pass() {
-    // A stream from the default impl should always have exactly one item.
     let tool = BlockingOk;
     let count = tool
         .execute(ToolCallContext::default(), EchoArgs { text: "n".into() })