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/git/checkout.rs

@@ -445,7 +445,7 @@ fn rehydrate_worktree_from_ref_inner(
 ) -> Result<WorktreeReport> {
     let dest_str = dest.to_string_lossy();
 
-    // A previously-disposed worktree can leave a stale registration for this
+    // An earlier-disposed worktree can leave a stale registration for this
     // path; prune it so re-adding the original `subagent-<id>` dir succeeds.
     snapshot_git(source_repo, &["worktree", "prune"], &[])?;
 

crates/codegen/kigi-fast-worktree/src/git/discovery.rs

@@ -145,7 +145,8 @@ mod tests {
         git_commit_all(temp.path(), "initial");
 
         let commit = get_head_commit(temp.path()).unwrap();
-        assert_eq!(commit.len(), 40); // SHA-1 hex string
+        // SHA-1 hex string
+        assert_eq!(commit.len(), 40);
         assert!(commit.chars().all(|c| c.is_ascii_hexdigit()));
     }
 }

crates/codegen/kigi-fast-worktree/src/git/mod.rs

@@ -1,7 +1,4 @@
 //! Git operations used by fast worktree creation.
-//!
-//! This module isolates git-specific functionality (worktree creation, status, index refresh)
-//! from filesystem copy logic and orchestration.
 
 pub(crate) mod checkout;
 pub(crate) mod discovery;

crates/codegen/kigi-fast-worktree/src/git/worktree.rs

@@ -1,12 +1,9 @@
-//! Git worktree operations.
-
 use std::path::Path;
 
 use anyhow::{Context, Result};
 
 use crate::git::checkout::git_command;
 
-/// Create a git worktree with `--no-checkout`. Blocking.
 pub(crate) fn worktree_add_no_checkout(source: &Path, dest: &str, git_ref: &str) -> Result<()> {
     let output = git_command()
         .current_dir(source)