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/agents_md.rs

@@ -239,7 +239,7 @@ mod tests {
         git2::Repository::init(path).unwrap();
     }
 
-    // ── find_agent_files unit tests ─────────────────────────────────
+    // find_agent_files unit tests
 
     #[test]
     fn find_agent_files_finds_agents_md() {
@@ -320,7 +320,7 @@ mod tests {
         assert!(files[1].to_string_lossy().contains("style.md"));
     }
 
-    // ── format_agents_md_section tests ──────────────────────────────
+    // format_agents_md_section tests
 
     #[test]
     fn format_agents_md_section_empty_returns_none() {
@@ -370,7 +370,7 @@ mod tests {
         );
     }
 
-    // ── Feature 2: Workspace user AGENTS.md via read_agents_config ───
+    // Feature 2: Workspace user AGENTS.md via read_agents_config
 
     #[tokio::test]
     async fn read_agents_config_includes_workspace_user_agents_md() {
@@ -537,7 +537,7 @@ mod tests {
         assert!(!section.contains("globs:"));
     }
 
-    // ── .claude/CLAUDE.md integration tests ─────────────────────────
+    // .claude/CLAUDE.md integration tests
 
     #[tokio::test]
     async fn read_agents_config_discovers_claude_subdir_claude_md() {