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/extensions/session_search.rs

@@ -22,7 +22,6 @@ use super::ExtResult;
 #[derive(Debug, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct SearchSessionsRequest {
-    /// The search query string.
     pub query: String,
     /// Optional workspace directory to scope results to.
     #[serde(default)]
@@ -67,7 +66,6 @@ pub struct SearchSessionHit {
     pub snippet: Option<String>,
 }
 
-/// Route `kigi/session/search` extension method calls.
 pub async fn handle(args: &acp::ExtRequest) -> ExtResult {
     match args.method.as_ref() {
         "kigi/session/search" => {
@@ -92,7 +90,6 @@ pub async fn handle(args: &acp::ExtRequest) -> ExtResult {
     }
 }
 
-/// Convert the internal response to the ACP-facing response.
 fn to_response(resp: SessionSearchResponse) -> SearchSessionsResponse {
     SearchSessionsResponse {
         results: resp