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-tools-api/src/slash_commands.rs

@@ -6,7 +6,6 @@
 /// keys `/loop` availability on this name.
 pub const SCHEDULER_CREATE_TOOL_NAME: &str = "scheduler_create";
 
-/// Usage hint shown when `/loop` is invoked with no arguments.
 pub fn loop_usage_message() -> &'static str {
     "Usage: /loop [interval] <prompt>\n\
      Example: /loop 30m check deploy status\n\
@@ -14,11 +13,9 @@ pub fn loop_usage_message() -> &'static str {
      Tell me how often it should run (e.g. 30m, 1 hour, every 2 days)."
 }
 
-/// Build the model instruction that `/loop` expands into for `args`.
-///
 /// The model, not brittle host parsing, turns the request into the
 /// `scheduler_create` interval, accepting every natural phrasing and erroring
-/// on bad input rather than silently defaulting. See [`loop_usage_message`].
+/// on bad input rather than silently defaulting.
 pub fn loop_schedule_instruction(args: &str) -> String {
     format!(
         "# /loop -- schedule a recurring prompt\n\n\