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-shell/tests/test_active_sessions_smoke.rs

@@ -20,19 +20,18 @@ fn full_lifecycle() {
     let pid = std::process::id();
     let sid = |s: &str| agent_client_protocol::SessionId::new(s);
 
-    // Start session, verify listed.
     register_in(r, session("s1", pid)).unwrap();
     assert_eq!(list_in(r).unwrap().len(), 1);
 
-    // Clean exit, verify gone.
     unregister_in(r, &sid("s1")).unwrap();
     assert!(list_in(r).unwrap().is_empty());
 
-    // Simulate crash (dead PID) + live session.
+    // 2_000_000_000 is not a live PID on this machine, simulating a crash.
     register_in(r, session("crashed", 2_000_000_000)).unwrap();
     register_in(r, session("alive", pid)).unwrap();
 
-    // Crash detection finds dead PID, keeps live one.
+    // collect_crashed_in also reaps the dead entry from the store, so
+    // list_in below drops from 2 to 1.
     let crashed = collect_crashed_in(r).unwrap();
     assert_eq!(crashed.len(), 1);
     assert_eq!(&*crashed[0].session_id.0, "crashed");