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-agent-lifecycle/src/send/contributors/session_lifecycle.rs

@@ -1,10 +1,9 @@
 use async_trait::async_trait;
 
-/// Input supplied when the host observes the session settling idle.
 pub struct SessionIdleInput;
 
 #[async_trait]
 pub trait SessionLifecycleContributor: Send + Sync {
-    /// Fired when the session settles idle (no running turn or queued work); the host owns the check.
+    /// Idle means no running turn and no queued work; the host owns that check.
     async fn on_session_idle(&self, _input: &SessionIdleInput) {}
 }