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-protocol/src/hook.rs

@@ -2,13 +2,10 @@
 
 use serde::{Deserialize, Serialize};
 
-/// Internally-tagged hook payload. New variants land alongside `Custom`,
-/// which keeps unknown future kinds round-trippable.
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 #[serde(tag = "type")]
 pub enum HookEvent {
-    /// Cancel an in-flight call. The owning `tool_call_id` travels in the
-    /// enclosing `hook` frame.
+    /// The `tool_call_id` this cancels travels in the enclosing `hook` frame.
     Cancel,
     Pause,
     Resume,