Regression - MINOR - Gitignore the override rundirs and note the load-bearing cleanup path

The sibling rundirs live inside the checkout, so without an ignore entry they show
as untracked in git status --porcelain and can spuriously mark golden provenance
dirty. Also state at both cleanup sites why rm targets rundir and not its parent.

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

Author: d-burg

.gitignore

@@ -38,3 +38,6 @@ scratch/
 
 # Local profiling scratch (one-off study scripts, not part of the package)
 profiling/
+
+# Override-case rundirs materialized beside their example by the regression harness
+.regress-override-*

regression-harness/src/runner.jl

@@ -525,6 +525,7 @@ function run_local(db::SQLite.DB, case_spec::CaseSpec, repo_root::String;
             rm(runinfo_file; force=true)
         end
         if rundir_is_temp && rundir !== nothing
+            # rundir sits beside the example; removing its parent would delete examples/ itself
             rm(rundir; recursive=true, force=true)
         end
     end
@@ -649,6 +650,7 @@ function run_at_commit(db::SQLite.DB, commit_hash::String, ref_name::String,
             rm(runinfo_file; force=true)
         end
         if rundir_is_temp && rundir !== nothing
+            # rundir sits beside the example; removing its parent would delete examples/ itself
             rm(rundir; recursive=true, force=true)
         end
         if own_worktree && worktree_path !== nothing