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/src/bundle.rs

@@ -1,10 +1,7 @@
 //! Location of the local subagent-content cache (`~/.kigi/bundled/`).
 //!
-//! Formerly this module managed a synced bundle of personas/roles/agents/
-//! skills fetched from the xAI cli-chat-proxy (`GET /v1/subagents/bundle`).
-//! That backend is gone; the directory remains a passive, locally-populated
-//! content root that role/persona discovery scans (see
-//! `config::resolve_*` discovery in `config/mod.rs`).
+//! A passive, locally-populated content root that role/persona discovery
+//! scans (see `config::resolve_*` discovery in `config/mod.rs`).
 
 use std::path::PathBuf;