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-hooks/tests/integration.rs

@@ -382,7 +382,8 @@ async fn hook_receives_env_vars() {
     // Verify env vars were received.
     let output = std::fs::read_to_string(&output_file).unwrap();
     assert!(output.contains("EVENT=pre_tool_use"), "output: {output}");
-    assert!(output.contains("NAME="), "output: {output}"); // auto-generated name
+    // auto-generated name
+    assert!(output.contains("NAME="), "output: {output}");
     assert!(output.contains("SESSION=sess-456"), "output: {output}");
 }