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/src/extensions/suggest/path_provider.rs

@@ -87,8 +87,6 @@ fn filter_executables(
     results
 }
 
-// --- PATH cache ---
-
 struct PathCacheInner {
     executables: Vec<String>,
     updated_at: Instant,
@@ -185,8 +183,6 @@ fn scan_path_from(path_var: &str) -> Vec<String> {
 mod tests {
     use super::*;
 
-    // --- extract_command_token ---
-
     fn cmd(prefix: &str) -> Option<(usize, String)> {
         extract_command_token(prefix).map(|t| (t.start, t.value))
     }
@@ -225,8 +221,8 @@ mod tests {
         assert_eq!(cmd("   "), None);
     }
 
-    /// A separator inside quotes is data, not a command position — the old
-    /// naive segment scan offered executables inside quoted strings.
+    /// A separator inside quotes is data, not a command position — a naive
+    /// segment scan would offer executables inside quoted strings.
     #[test]
     fn none_inside_quoted_data() {
         assert_eq!(cmd("echo \"x | gr"), None);
@@ -239,8 +235,6 @@ mod tests {
         assert_eq!(cmd("> lo"), None);
     }
 
-    // --- filter_executables ---
-
     fn tok(prefix: &str) -> CurrentToken {
         parse_current_token(prefix)
     }
@@ -326,8 +320,6 @@ mod tests {
         assert_eq!(results[0].insert_text, "\"grep\"");
     }
 
-    // --- scan_path_from ---
-
     #[test]
     fn scan_nonexistent_dir() {
         assert!(scan_path_from("/nonexistent/path/that/doesnt/exist").is_empty());