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/plugins/git_install.rs

@@ -561,7 +561,7 @@ pub struct UpdateResult {
 
 /// Status of an update attempt.
 pub enum UpdateStatus {
-    /// Repo was updated successfully.
+    /// Repo updated successfully.
     Updated(UpdateResult),
     /// Repo is pinned to a tag or commit — no automatic update.
     Pinned { ref_name: String },
@@ -633,7 +633,7 @@ pub fn update_repo(repo_key: &str, repo: &InstalledRepo) -> Result<UpdateStatus,
             let new_commit = read_head_commit(repo_path);
             let changed = old_commit.as_deref() != new_commit.as_deref();
 
-            // Re-discover plugins (new ones may have been added)
+            // Re-discover plugins: the pull may bring new ones
             let plugins = discover_plugins_in_dir(repo_path, subdir.as_deref())?;
 
             Ok(UpdateStatus::Updated(UpdateResult {