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-fast-worktree/src/lib.rs

@@ -52,11 +52,10 @@ pub use sync::{SourceDirtyState, SyncReport, WorktreeSync, collect_source_dirty_
 #[cfg(target_os = "linux")]
 pub use worktree::execute::cleanup_snapshot_git_state;
 
-/// Count the number of tracked files in a git repository's index.
+/// Count the tracked files in a git repository's index.
 ///
-/// Reads the index header via `gix`, which contains the entry count — this
-/// is an O(1) read (no directory walk). Useful for deciding whether a repo
-/// is large enough to benefit from worktree pooling.
+/// Reads the entry count out of the index header via `gix`, so this is an
+/// O(1) read with no directory walk.
 pub fn count_tracked_files(repo_path: &std::path::Path) -> anyhow::Result<usize> {
     let repo = gix::discover(repo_path)
         .map_err(|e| anyhow::anyhow!("failed to discover git repo: {e}"))?;