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/loader.rs

@@ -84,7 +84,6 @@ pub fn load_from_disk() -> std::io::Result<toml::Value> {
     load_user_config_layer(user_kigi_home().as_deref(), "config.toml")
 }
 
-/// Managed config filename, shared by the loaders in this module.
 pub const MANAGED_CONFIG_FILENAME: &str = "managed_config.toml";
 
 pub fn load_managed_config() -> std::io::Result<toml::Value> {
@@ -111,7 +110,6 @@ pub fn load_system_managed_config() -> std::io::Result<toml::Value> {
     Ok(v)
 }
 
-/// One managed-config layer: the parsed TOML and the file it came from.
 #[derive(Debug, Clone)]
 pub struct ManagedConfigLayer {
     pub value: toml::Value,
@@ -377,7 +375,6 @@ pub struct CampaignsState {
     pub dismissed_ids: Vec<String>,
 }
 
-/// Path to `$KIGI_SHARE_DIR/campaigns_state.json` under `home`.
 pub fn campaigns_state_path(home: &std::path::Path) -> std::path::PathBuf {
     home.join(CAMPAIGNS_STATE_FILE)
 }