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/tests/git_contention_e2e.rs

@@ -52,8 +52,6 @@ fn git(dir: &Path, args: &[&str]) -> String {
     kigi_test_utils::git::run_git(dir, args)
 }
 
-// ── scan counting ─────────────────────────────────────────────────────────
-
 use kigi_hunk_tracker::{REFRESH_SCAN_LOG_PREFIX, REFRESH_SKIP_LOG_PREFIX};
 use kigi_test_utils::tracing_capture::MessagePrefixCounter;
 
@@ -84,8 +82,6 @@ fn install_global_scan_counter() -> ScanCounter {
     )
 }
 
-// ── repo fixture ──────────────────────────────────────────────────────────
-
 /// Committed tree of ~`files` files plus a `feature` branch with `picks`
 /// one-file commits and an advanced base branch, `feature` checked out.
 /// Returns the repo dir and the base branch name.
@@ -105,8 +101,6 @@ fn build_repo(files: usize, picks: usize) -> (TempDir, String) {
     (dir, base)
 }
 
-// ── scripted responses (chat-completions SSE) ────────────────────────────
-
 fn chat_chunk(delta: Value, finish_reason: Value) -> SseEvent {
     SseEvent::data(
         json!({
@@ -151,8 +145,6 @@ fn text_sse(text: &str) -> ScriptedResponse {
     ScriptedResponse::sse(events)
 }
 
-// ── client ────────────────────────────────────────────────────────────────
-
 /// Auto-approves permissions (AllowOnce preferred) and drops notifications.
 struct AutoApproveClient;
 
@@ -181,8 +173,6 @@ impl acp::Client for AutoApproveClient {
     }
 }
 
-// ── one full agent run ────────────────────────────────────────────────────
-
 struct RunStats {
     scans: usize,
     skips: usize,