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-chat-state/src/compaction_utils.rs

@@ -299,7 +299,7 @@ pub fn extract_last_user_query(conversation: &[ConversationItem]) -> Option<Stri
         .map(|item| extract_user_query(&item.text_content()))
         .filter(|q| !q.is_empty())
 }
-/// The continuation prompt added to the conversation after auto-compaction.
+/// The continuation prompt appended to the conversation after auto-compaction.
 ///
 /// Stored here (rather than only in `kigi-shell`) so that query-extraction
 /// helpers in this crate can recognise and exclude it from "real user prompt"
@@ -666,7 +666,7 @@ pub fn format_compact_summary(summary: &str) -> String {
 /// A markdown "**Analysis**"-style header has no opening `<analysis>` tag for
 /// step 1 to catch; it ends at an orphan `</analysis>`. Everything up to and
 /// including the *last* `</analysis>` is dropped, so a scratchpad that itself
-/// quotes `</analysis>` mid-reasoning is still removed whole. The peel is
+/// quotes `</analysis>` mid-reasoning is still stripped whole. The peel is
 /// skipped when the block already starts with a numbered section — including a
 /// markdown-decorated one like `## 1.` or `**1.**` — so a `</analysis>` merely
 /// echoed inside a real section never truncates the summary. Any leftover