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/compaction.rs

@@ -1,24 +1,18 @@
 //! Compaction policy — threshold, model, and memory flush configuration.
 
-/// Session-level compaction policy.
-///
-/// Controls when and how the session's conversation is compacted
-/// to free up context window space, and whether a memory flush
-/// runs before each compaction.
+/// Controls when and how the session's conversation is compacted to free up
+/// context window space, and whether a memory flush runs before each compaction.
 #[derive(Debug, Clone)]
 pub struct CompactionPolicy {
     /// Percentage of context window that triggers auto-compaction.
-    /// E.g., 85 means compact when 85% of the context window is used.
     pub auto_compact_threshold_percent: u32,
 
-    /// Model to use for generating the compaction summary.
-    /// None = use the session's current model.
+    /// `None` uses the session's current model.
     pub compact_model: Option<String>,
 
-    /// Whether to run a memory flush turn before each compaction.
-    /// When enabled, the session actor asks the model to summarize
-    /// important information from the conversation before it's compacted.
-    /// Requires the memory system to be enabled.
+    /// Run a memory flush turn before each compaction: the session actor asks
+    /// the model to summarize important information from the conversation
+    /// before it is discarded. Requires the memory system to be enabled.
     pub memory_flush_enabled: bool,
 
     /// Per-compaction wall-clock budget (seconds); a generation exceeding it is
@@ -27,9 +21,9 @@ pub struct CompactionPolicy {
 
     /// Prefire two-pass compaction: when usage approaches the threshold,
     /// speculatively summarize the history prefix in the background (pass 1);
-    /// at compaction, summarize NOTE₁ + the recent tail (pass 2). Resolved from
-    /// config (`two_pass_compaction` flag) at session build; `false` keeps the
-    /// legacy single-pass path. Default `false` (real sessions set it from config).
+    /// at compaction, summarize NOTE₁ + the recent tail (pass 2). `false`
+    /// selects the single-pass path. Real sessions resolve this from the
+    /// `two_pass_compaction` config flag at session build.
     pub two_pass_enabled: bool,
 }