fix: post-squash compile errors reclassify as semantic merge conflicts

When deterministic-merge produces a clean git squash but the post-squash
compile fails (typical when master gained a Stage payload field after the
feature branch forked — e.g. story 1018 hit `error[E0063]: missing field
plan` after 1010's PlanState landed), the failure is morally a merge
conflict that git's diff3 missed: the conflicting literal lives in a
different file from the type definition that changed on master. Routing
it as GatesFailed left mergemaster idle and the story stuck.

Changes:
- gates.rs GateFailureKind::classify: detect rustc compile errors
  (`error[E\d+]`) as Build instead of falling through to Test. Clippy
  errors (`error[clippy::...]`) still classify as Lint.
- agents/merge/mod.rs: new MergeResult::to_merge_failure_kind() method.
  GateFailure with failure_kind=Build maps to ConflictDetected (so the
  existing 998 subscriber auto-spawns mergemaster). Other gate failures
  stay GatesFailed.
- agents/pool/pipeline/merge/runner.rs: replace the inline match with a
  call to the new method.

Tests: 6 new unit tests covering the classifier branch and every
to_merge_failure_kind arm. All 2932 tests pass.

server/src/agents/gates.rs

@@ -44,6 +44,13 @@ impl GateFailureKind {
             || output.contains("missing_doc_comments")
         {
             GateFailureKind::Lint
+        } else if output.contains("error[E") {
+            // rustc compile errors (e.g. `error[E0063]: missing field`).
+            // When this appears in the post-squash gate run, it almost always
+            // signals cross-merge breakage — master gained a field/variant the
+            // feature branch's code does not match. Mergemaster handles the
+            // recovery in its ConflictDetected path.
+            GateFailureKind::Build
         } else {
             GateFailureKind::Test
         }
@@ -880,6 +887,29 @@ mod tests {
         );
     }
 
+    #[test]
+    fn classify_build_from_rustc_compile_error() {
+        // Post-squash compile errors (typical when master drifts under a feature
+        // branch — e.g. story 1018 hit `error[E0063]: missing field` after
+        // master gained a Stage::Coding field the feature branch did not set).
+        assert_eq!(
+            GateFailureKind::classify(
+                "error[E0063]: missing field `plan` in initializer of `Stage`\n   --> server/src/foo.rs:166:20"
+            ),
+            GateFailureKind::Build
+        );
+    }
+
+    #[test]
+    fn classify_build_does_not_misfire_on_clippy_error() {
+        // Clippy errors look like `error[clippy::name]` and must remain Lint,
+        // not Build, because the `error[E` prefix would otherwise overlap.
+        assert_eq!(
+            GateFailureKind::classify("error[clippy::unused_variable]: unused variable `x`"),
+            GateFailureKind::Lint
+        );
+    }
+
     #[test]
     fn classify_test_failure_for_unrecognised_output() {
         assert_eq!(