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/session/helpers/memory_context.rs

@@ -186,10 +186,6 @@ mod tests {
         assert!(output.contains("score: 0.70"));
     }
 
-    // -----------------------------------------------------------------------
-    // conversation_has_memory_context (idempotency guard) tests
-    // -----------------------------------------------------------------------
-
     fn sample_result() -> MemorySearchResult {
         MemorySearchResult {
             chunk_id: "test:0".into(),
@@ -237,10 +233,6 @@ mod tests {
         assert!(!conversation_has_memory_context(&[]));
     }
 
-    // -----------------------------------------------------------------------
-    // staleness annotation tests
-    // -----------------------------------------------------------------------
-
     #[test]
     fn test_staleness_shown_for_old_session_result() {
         let now = std::time::SystemTime::now()
@@ -287,10 +279,6 @@ mod tests {
         );
     }
 
-    // -----------------------------------------------------------------------
-    // is_greeting tests
-    // -----------------------------------------------------------------------
-
     #[test]
     fn test_greeting_detection() {
         assert!(is_greeting("hi"));
@@ -309,10 +297,6 @@ mod tests {
         assert!(!is_greeting("hi there, can you help me with something"));
     }
 
-    // -----------------------------------------------------------------------
-    // Injection counter semantics tests
-    // -----------------------------------------------------------------------
-
     /// `format_memory_reminder` returns `None` for an empty result list.
     ///
     /// This is the key invariant for the `memory_injection_count` contract: