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-workspace/src/session/tool_config.rs

@@ -315,8 +315,7 @@ impl SessionContextFactory for WorkspaceSessionContextFactory {
         IDS.clone()
     }
 }
-/// Build web fetch config. Enabled with default params unless
-/// `KIGI_DISABLE_WEB_FETCH=1` is set.
+/// Enabled with default params unless `KIGI_DISABLE_WEB_FETCH=1` is set.
 fn build_web_fetch_config() -> kigi_tools::implementations::kigi::web_fetch::WebFetchConfig {
     use kigi_tools::implementations::kigi::web_fetch::{WebFetchConfig, WebFetchParams};
     if std::env::var("KIGI_DISABLE_WEB_FETCH").is_ok_and(|v| v == "1" || v == "true") {