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/prompt_meta.rs

@@ -14,14 +14,12 @@ pub struct PromptBlockMeta {
 }
 
 impl PromptBlockMeta {
-    /// Create meta for a direct bash command.
     pub fn bash(command: impl Into<String>) -> Self {
         Self {
             bash_command: Some(command.into()),
         }
     }
 
-    /// Try to parse from a freeform `_meta` map.
     pub fn from_value(value: &agent_client_protocol::Meta) -> Option<Self> {
         serde_json::from_value(serde_json::Value::Object(value.clone())).ok()
     }