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-agent/src/error.rs

@@ -1,36 +1,29 @@
 //! Error types for agent construction.
 
-/// Errors that can occur during Agent construction.
 #[derive(Debug, thiserror::Error)]
 pub enum AgentBuildError {
-    /// Failed to parse the agent definition file (bad YAML frontmatter,
-    /// missing closing `---`, or invalid Markdown structure).
+    /// Bad YAML frontmatter, a missing closing `---`, or invalid Markdown
+    /// structure in the definition file.
     #[error("failed to parse agent definition: {0}")]
     ParseError(String),
 
-    /// Required fields are missing from the definition (name, description).
     #[error("missing required field in agent definition: {0}")]
     MissingField(String),
 
-    /// A tool name override references a tool that doesn't exist in the
-    /// registry (typo in the definition's `toolNameOverrides`).
+    /// Usually a typo in the definition's `toolNameOverrides`.
     #[error("tool name override references nonexistent tool '{0}'")]
     UnknownToolOverride(String),
 
-    /// IO error during AGENTS.md or skills discovery.
     #[error("IO error during agent construction: {0}")]
     IoError(#[from] std::io::Error),
 
-    /// MiniJinja template rendering failed (extend or full mode).
-    /// Includes line numbers and context from the template.
+    /// Carries template line numbers and surrounding context.
     #[error("template rendering error: {0}")]
     MiniJinjaError(#[from] minijinja::Error),
 
-    /// Tool registry error (e.g., unsatisfied requirements during finalization).
     #[error("tool error: {0}")]
     ToolError(String),
 
-    /// A configuration value is present but invalid (e.g. `max_turns = 0`).
     #[error("invalid configuration: {0}")]
     InvalidConfig(String),
 }