tools (volt): ruff -F,E9 3 -> 0 -- an unused import, a placeholder-free f-string and a dead variable out of the instruments; 30 captured outputs unchanged (429 cells still green)

ruff check --select F,E9 tools/ -> 3 errors at 960b852:
  checkin_check.py:421 F541  f-string with no placeholder
  checkin_check.py:556 F841  jd assigned, never read (dead residue of an unwritten
                             malformed-JSON refusal cell)
  queue_append_audit.py:73 F401  re imported, never used (no 're.' in the file)
Fix: 3 lines removed, 1 line changed, nothing added (numstat 1/2 + 0/1).

Behaviour claim measured, not asserted: all 14 tools' --selftest stdout+stderr
captured before and after (28 files) plus checkin_check.py --dir checkins and
queue_append_audit.py --guard --to HEAD (2 files) -- 30 artifacts, 0 differ after
normalising only checkin_check's mkdtemp suffix. 14 tools / 429 cells / 429 pass
on both sides; verify.py 3/3 PASS rc 0.

Not enforced by the gate: verify.py has no static-check leg, so this property is
observed here only. Adding ruff to the gate would make it a hard dependency
(host without ruff -> UNGRADABLE -> rc 2), so no leg and no new tool.

tools/checkin_check.py

@@ -418,7 +418,7 @@ def print_report(rep, show_gaps: bool, fail: bool):
     adv = rep["advisory"]
     print(f"\nadvisory (never rc 1): {adv['missing_headings']} of {rep['checked']} "
           f"files lack a Did/Blocked/Next/Ask heading")
-    print(f"  -- A1: the four-heading rule postdates the archive; grade it where it applies")
+    print("  -- A1: the four-heading rule postdates the archive; grade it where it applies")
     if show_gaps:
         print(f"\n{'handle':<12} {'files':>5} {'days':>5} {'first':>10} {'last':>10} "
               f"{'span':>4} {'miss':>4} {'gap':>4} {'idle':>4}")
@@ -553,7 +553,6 @@ def selftest():
         cell("c20 --fail exits 1 when a fault exists", rc_fail == 1, str(rc_fail))
 
         # refusals: rc 2, never a traceback, never 1
-        jd = os.path.join(td, "j.json")
         miss = os.path.join(td, "no-such-dir")
         rc_missing = main(["--dir", miss, "--roster", "volt"])
         cell("c21 a missing checkins dir refuses at rc 2", rc_missing == 2, str(rc_missing))

tools/queue_append_audit.py

@@ -70,7 +70,6 @@ import argparse
 import io
 import json
 import os
-import re
 import subprocess
 import sys
 import tempfile