swarm-controller: the review classifier reads a diff with 25 lines of context, not 200

The wide diff's context drops from 200 to 25 lines, so a block's
enclosing gate stays in view when it opens within 25 lines above the
change (the forge's own diff carries 3). The fallback note, the doc and
the gate fixture (now 20 lines out, not 47) follow.

swarm-controller/src/forge/pull_diff.rs

@@ -10,9 +10,10 @@ use std::time::Duration;
 
 use similar::TextDiff;
 
-/// Lines of context around each change. In this repo's Nix modules a block's
-/// enclosing `config = lib.mkIf …` sits up to 191 lines above it.
-pub const CONTEXT_LINES: usize = 200;
+/// Lines of context around each change. A block's enclosing
+/// `config = lib.mkIf …` is in view when it opens within this many lines above
+/// the block.
+pub const CONTEXT_LINES: usize = 25;
 
 /// The largest wide diff sent, in bytes, about 64k tokens of code. A larger
 /// one gives way to the forge's `.diff`.
@@ -172,8 +173,8 @@ mod tests {
             change("nix/host-modules/glue-nats-wireguard.nix", None, Some(glue)),
             change(
                 "nix/host-modules/swarm-nats.nix",
-                Some(&module(47, true)),
-                Some(&module(47, false)),
+                Some(&module(20, true)),
+                Some(&module(20, false)),
             ),
         ]
     }

swarm-controller/src/review_classifier/review.rs

@@ -2033,7 +2033,7 @@ mod tests {
         let body = narrowed(reason(), true).await;
         assert!(
             body.ends_with(
-                "\nThe classifier read the forge's diff, with 3 lines of context, not 200: \
+                "\nThe classifier read the forge's diff, with 3 lines of context, not 25: \
                  the PR changes 101 files.\n"
             ),
             "{body}"