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/src/prompt/workspace_user.rs

@@ -48,7 +48,7 @@ mod tests {
     use super::*;
     use std::fs;
 
-    // ── resolve_workspace_user_dir (pure, no env vars) ───────────────
+    // resolve_workspace_user_dir (pure, no env vars)
 
     #[test]
     fn resolve_returns_none_for_empty_root() {
@@ -126,7 +126,7 @@ mod tests {
         assert_eq!(result, Some(user_dir));
     }
 
-    // ── workspace_user_relpath ───────────────────────────────────────
+    // workspace_user_relpath
 
     #[test]
     fn bare_username_is_nested_under_x() {