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-tui/src/app/agent.rs

@@ -52,7 +52,6 @@ pub struct QueuedPrompt {
     pub id: u64,
     /// The prompt text (or command text, e.g. "/compact").
     pub text: String,
-    /// Whether this is a prompt or a slash command.
     pub kind: QueueEntryKind,
     /// Optional separate payload for the wire. When `Some`, this is sent
     /// instead of `text`. Used for skill injection where the display
@@ -117,10 +116,6 @@ impl QueuedPrompt {
 /// which execute immediately without going through the queue or agent.
 ///
 /// Each variant carries the data needed for execution and display.
-/// Using an enum instead of a String gives us:
-/// - Type safety (can't misspell command names)
-/// - Variant-specific data (e.g., `/model` would carry target model)
-/// - Proper rendering per command type
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum AgentCommand {
     /// `/compact` — compact conversation history.
@@ -163,7 +158,6 @@ pub const BG_TASK_MAX_STDOUT: usize = 10 * 1024 * 1024;
 /// How long to wait for a kill response before auto-clearing `pending_kill`
 /// so the user can retry. Applied to both bg tasks and subagents.
 pub const PENDING_KILL_TIMEOUT_SECS: u64 = 10;
-/// Status of a background task.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum BgTaskStatus {
     /// Currently running.