search-with-proof: ecosystem-aware build recipes and stderr in build failures

The agent passes a manifest file or project directory; pip uses -r for
requirements files and the directory for pyproject/setup, npm ci/install runs in
the directory, composer/maven/gradle likewise. Build and verify failures carry
phase, exit code and a sanitised stderr tail.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

benchmarks/ax/search_verify.py

@@ -110,4 +110,7 @@ class AXSideDispatcher:
             max(r.scratch_peak_bytes for r in records),
             isolation_mode="task-boundary",
             chromium_no_sandbox=any(r.chromium_no_sandbox for r in records),
+            phase=last.phase,
+            exit_code=last.exit_code,
+            stderr=last.stderr,
         )