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-config/src/paths.rs

@@ -146,9 +146,6 @@ pub fn decode_cwd_from_dirname(dir: &std::path::Path) -> Option<String> {
         .map(|s| s.trim().to_string())
 }
 
-/// Build the CWD-level session directory path:
-/// `kigi_home()/sessions/{encode_cwd_dirname(cwd)}`.
-///
 /// Does **not** create the directory on disk — use [`ensure_sessions_cwd_dir`]
 /// when the directory must exist.
 pub fn sessions_cwd_dir(cwd: &str) -> PathBuf {
@@ -181,10 +178,8 @@ pub fn ensure_sessions_cwd_dir(cwd: &str) -> std::io::Result<PathBuf> {
     Ok(dir)
 }
 
-/// Generate a URL-safe slug from a string.
-///
-/// Lowercases, replaces non-alphanumeric chars with `-`, collapses
-/// consecutive dashes, and truncates to `max_len` characters.
+/// Output is ASCII-only, so `max_len` bounds the result in bytes as well as
+/// chars — [`encode_cwd_dirname`] relies on that for its length guarantee.
 fn slugify(input: &str, max_len: usize) -> String {
     let mut result = String::with_capacity(input.len());
     let mut prev_dash = false;