fix(bin): contain and reap test fixtures instead of leaking them into /tmp

.agents/skills/bootstrap-diagnostics/SKILL.md

@@ -46,8 +46,9 @@ When any diagnostic needs captain attention, report the plain consequence and re
   Keep each affected poll unavailable, inspect the named private state path, and do not bypass the migration or execute a quarantined artifact; a completed safe-scan marker allows unrelated authenticated polls to continue while private repair remains pending.
 - `SCRATCH_SWEEP: <name>: skipped: <reason>` - the temp-root scratch sweep found a stale orphaned scratch directory but deliberately left it in place, either because removal failed or because it does not look like scratch at all (`looks like a firstmate home` means a home is sitting in the temp root and must be moved or removed by hand, never swept).
   The shared temp filesystem is small and every home on the host writes into it, so a leak that keeps being skipped eventually fills it and makes every command on the machine fail silently on temp writes; clear the named directory yourself once you have confirmed nothing is using it.
-- `SCRATCH_SWEEP: <temp root>: skipped: <reason>` - the sweep could not run to completion for the whole root: either no open-handle check was available (install `lsof`, or check why procfs or the temp root is unreadable) or the sweep's time budget ran out with candidates left.
+- `SCRATCH_SWEEP: <temp root>: skipped: <reason>` - the sweep could not run to completion for the whole root: no open-handle check was available (install `lsof`, or check why procfs or the temp root is unreadable), the sweep's time budget ran out with candidates left, or a firstmate home's records could not be read so the sweep refused before examining anything.
   A budget line is self-healing across sessions and needs attention only if the deferred count keeps growing; a missing open-handle check disables reclamation entirely and does need fixing.
+  An unreadable-records line means nothing at all was reclaimed this session, so never report a completed sweep after one: repair the records the reason names (a home's task records under `state/`, or the home directory or secondmate registry that decides which homes are read) rather than clearing the temp root by hand, because a live task's recorded scratch is exactly what could not be ruled out.
 - `SECONDMATE_SYNC: secondmate <id>: skipped: <reason>` - the local-HEAD secondmate sync left a live secondmate home on its existing checkout because the home was dirty, diverged, unsafe, on the wrong branch, missing the primary target commit, or otherwise not fast-forwardable, or because inherited local-material propagation failed; bootstrap continued, but inspect the reason because the secondmate's tracked instructions, inherited settings, or shared captain preferences may be stale after a primary update.
 - `SECONDMATE_LIVENESS: secondmate <id>: skipped: <reason>|respawn failed after <cause>: <reason>` - the session-start liveness sweep could not guarantee that the registered secondmate is running a real agent process.
   Investigate the reason because that secondmate is not guaranteed live.

CONTRIBUTING.md

@@ -97,6 +97,9 @@ Family selection is the ordinary local path; `--all` is deliberate full regressi
 CI owns broad regression across required portable parallel shards, the portable serial lane, the Herdr lane, lint, invariants, the coverage guard, and stock macOS Bash compatibility in [`.github/workflows/ci.yml`](.github/workflows/ci.yml).
 Use `bin/fm-test-run.sh --help` for lane names, `--jobs` rules, and required gate-skip flags when reproducing a lane locally.
 Discover tests by listing `tests/*.test.sh`: each is a self-contained bash script named `<subject>.test.sh`, and its header comment describes what it covers, so pass one to `bin/fm-test-run.sh` to focus on a subject with canonical timing output.
+The runner is also the single owner of fixture cleanup: it gives every executed script a private `TMPDIR` and removes it when that script finishes, on an interrupted run as well as a normal one, and it reaps fixture directories orphaned by earlier killed runs, so a new test needs no temp-root `EXIT` trap of its own.
+That matters because `/tmp` is a tmpfs on some hosts, where a leaked fixture is leaked RAM rather than disk.
+`bin/fm-tmp-sweep.sh` is the single owner of which orphaned scratch directories are safe to remove, for both the runner's reap and the session-start sweep, and the runner's header owns its own test-only environment seams.
 Tests that need a real optional backend or an explicit opt-in (real herdr/zellij/cmux smoke tests, the live Pi regression) skip themselves and print the tool or environment gate needed to enable them, so the portable suite remains safe on machines without those tools.
 The [Herdr backend guide](docs/herdr-backend.md#destructive-lab-safety) owns the lane's isolation boundary, while [runtime backend verification](docs/verification/runtime-backends.md#herdr) owns active empirical evidence; live harness credential tests remain opt-in.
 

bin/fm-spawn.sh

@@ -1325,7 +1325,20 @@ fi
 # Nested (not a bare /tmp/fm-<id>/gotmp) so other per-task temp can live alongside
 # later, and teardown cleans one deterministic path. GOTMPDIR (not TMPDIR) is the
 # targeted knob: TMPDIR is too broad (affects every program's temp, not just Go's).
-TASK_TMP="/tmp/fm-$ID"
+#
+# FM_TASK_TMP_ROOT relocates that root and is unset in normal operation, so the
+# real path is unchanged. bin/fm-test-run.sh sets it to the executed script's
+# private temp root: a test that drives a real spawn would otherwise leave a
+# /tmp/fm-<fixture id>/ behind with no task and no teardown to remove it, and on
+# a tmpfs /tmp that residue is held in RAM. It must be absolute: this path is
+# recorded as tasktmp= and reaches an unguarded rm -rf in bin/fm-teardown.sh,
+# which would resolve a relative value against whatever directory teardown
+# happens to run in.
+case "${FM_TASK_TMP_ROOT:-/tmp}" in
+  /*) ;;
+  *) echo "error: FM_TASK_TMP_ROOT must be an absolute path, got '$FM_TASK_TMP_ROOT'" >&2; exit 1 ;;
+esac
+TASK_TMP="${FM_TASK_TMP_ROOT:-/tmp}/fm-$ID"
 mkdir -p "$TASK_TMP/gotmp"
 
 # Per-harness turn-end hook where enabled: a file that touches