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/session/commands.rs

@@ -1,15 +1,13 @@
 //! Session actor command enum and associated public types.
 //!
-//! `SessionCommand` defines the message protocol used to drive a session
-//! actor. It was extracted from `acp_session.rs` to keep the actor
-//! implementation focused on behaviour.
+//! `SessionCommand` defines the message protocol used to drive a session actor.
 use super::acp_types::*;
 use super::plan_mode::PromptMode;
 use crate::extensions::notification::SessionNotification;
 use crate::session::signals::TurnDeltaSnapshot;
 use agent_client_protocol as acp;
 use tokio::sync::oneshot;
-/// Structured context for a cancelled turn, replacing stringly-typed JSON.
+/// Structured context for a cancelled turn.
 #[derive(Debug, Clone, Default, serde::Deserialize)]
 pub struct CancellationContext {
     pub tool_name: Option<String>,
@@ -56,10 +54,7 @@ pub struct PromptTurnOk {
     pub structured_output: Option<Result<serde_json::Value, String>>,
     pub usage: Option<crate::extensions::notification::PromptUsage>,
 }
-/// Result of a prompt turn, containing the stop reason, accumulated token count,
-/// and an optional turn-end signals snapshot (for trace metadata enrichment).
 pub type PromptTurnResult = Result<PromptTurnOk, acp::Error>;
-/// Convenience: successful end-of-turn result.
 pub(crate) fn ok_end_turn(tokens: u64, snapshot: Option<TurnDeltaSnapshot>) -> PromptTurnResult {
     Ok(PromptTurnOk {
         stop_reason: acp::StopReason::EndTurn,