Lighten the comment load

Drops porting-history narratives (dodrenett), superseded-behavior
explanations, and restatements of what the next line does. Constraint
notes (fail-closed policies, CAS semantics, cascade behavior, id
uniqueness) stay, just shorter. No code changes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/chain.rs

@@ -1,11 +1,6 @@
-//! SHA-256 hashed DAG chain, ported from dodrenett's lib/challenge.ts.
-//! Kept deliberately plain: every submission hashes in its parent(s), but
-//! nothing here produces more than one parent yet - that only starts
-//! mattering once something (an LLM, or a human) posts follow-up
-//! questions back into a visitor's chain instead of the fixed content in
-//! ../portal-content. The DAG-shaped `parent_hashes` field exists so that
-//! can slot in later without a schema change; the hashing itself doesn't
-//! need to.
+//! SHA-256 hashed answer chain. `parent_hashes` is DAG-shaped, but
+//! nothing produces more than one parent yet - the shape is reserved
+//! for follow-up questions posted back into a visitor's chain.
 #![cfg(feature = "ssr")]
 
 use sha2::{Digest, Sha256};