chore: add harness gitignore rules (missing since this branch never got them on resume)

.gitignore

@@ -14,3 +14,20 @@ reviews/
 # cross-version drift. Untracked, that guarantee is gone — a fresh clone has no
 # fixtures at all — so the task's Verify section needs rewriting to match.
 **/tests/fixtures/
+# agent-harness: build artifacts and vendored archives never belong in source control
+*.tar.gz
+*.tgz
+*.crate
+*.zip
+*.bin
+*.whl
+vendor/
+node_modules/
+
+# agent-harness: task/phase completion sentinel files, harness bookkeeping only
+.task-result-*
+.phase-result-*
+.stage-done-*
+
+# agent-harness: PLAN.md is per-task planner scratch state, never a deliverable
+PLAN.md