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-workspace/src/activity.rs

@@ -1244,7 +1244,7 @@ mod tests {
             let old = now_ms() - SESSION_IDLE_PRUNE_MS - 1000;
             session.idle_since_ms.store(old, Ordering::Relaxed);
         }
-        // Prune should remove the session and its call_to_session entries.
+        // Prune should drop the session and its call_to_session entries.
         t.known_sessions();
         assert!(!t.sessions.contains_key("stale"));
         // Verify call_to_session was cleaned (no dangling entries).
@@ -1449,7 +1449,7 @@ mod tests {
         );
     }
 
-    // ── events.jsonl emission ─────────────────────────────────
+    // events.jsonl emission
 
     /// Build a tracker whose event sink points at a fresh tempdir, with the
     /// `events.jsonl` writer for `session` pre-opened (as turn-start would do).