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/mod.rs

@@ -1,15 +1,9 @@
-//! Plugin system — discover, load, and manage plugins (including compat layouts).
+//! Plugin discovery, loading, and registry.
 //!
 //! A plugin is a self-contained directory that bundles skills, agents,
-//! MCP server configs, and hooks into a namespaced unit.  Plugins can
+//! MCP server configs, and hooks into a namespaced unit. Plugins can
 //! live under `~/.kigi/plugins/`, `.kigi/plugins/` (project-level),
 //! or be passed via `--plugin-dir` on the CLI.
-//!
-//! This module handles:
-//! - `manifest` — parsing `plugin.json` manifests
-//! - `discovery` — scanning the filesystem for plugin directories
-//! - `trust` — project-plugin trust management
-//! - `registry` — in-memory registry of active plugins
 
 pub mod discovery;
 pub mod git_install;