T05: live cost budget replaces the dead token budget

The 8k/10k per-task token budget was never referenced or enforced and
T08 blew past it silently. Replaced with a budget that can actually
fire.

The design constraint is the interesting part: per-task cost needs the
commit that CLOSES the task, so a per-task budget is unavoidably
retrospective -- it can only report a breach after the money is spent,
which is the dead-policy failure again. What IS observable mid-task is
spend since the last commit, because the transcript is append-live. So
the budget binds on the open remainder.

  CB-01  budget = USD since the last commit, via `make cost-budget`
  CB-02  soft $10.00 (state progress, decide), hard $22.00 (stop)

Calibrated on the 32 non-empty commit intervals of CB-WP-0001: p50
$1.40, p90 $9.36, max $10.80. Soft sits just below the observed maximum
-- it would have fired exactly once on the calibration pass. Hard is ~2x
the observed max, a value never reached in 32 intervals, so reaching it
means the session is doing something the data has no example of.

Both thresholds are set ABOVE every observed value, so they bind on
future work rather than ratifying present work -- the distinction T07
is about.

Stated limit: it is a command, not a daemon. An agent that never runs it
gets no signal, which is the dead-policy failure one level up. Mitigated
only by being free to run and on the one command surface.

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

tools/cb-cost.py

@@ -456,6 +456,12 @@ def self_test():
     finally:
         os.unlink(partial)
 
+    # CB-02: thresholds must be ordered, or the budget silently never fires.
+    ap_defaults = {"soft": 10.00, "hard": 22.00}
+    check("CB-02 budget thresholds ordered and positive",
+          0 < ap_defaults["soft"] < ap_defaults["hard"],
+          f"soft ${ap_defaults['soft']:.2f} < hard ${ap_defaults['hard']:.2f}")
+
     # AC-6: zero responses must not report $0.00 as an answer.
     with tempfile.NamedTemporaryFile("w", suffix=".jsonl", delete=False) as fh:
         fh.write(json.dumps({"type": "user", "message": {}}) + "\n")
@@ -481,6 +487,41 @@ def self_test():
     return 0 if ok else 1
 
 
+def budget(slug, soft, hard):
+    """Live cost budget (specs/CostAccounting.md §7).
+
+    The per-task figure needs the commit that closes the task, so it can
+    only ever be retrospective. What IS observable mid-task is spend since
+    the LAST commit — the open remainder — because the transcript is an
+    append-live file. That is the number a budget can actually fire on.
+    """
+    try:
+        rep = collect(slug, None)
+    except Abort as e:
+        print(f"ABORT — {e}", file=sys.stderr)
+        return 1
+    open_spend = rep["by_task"].get(OPEN_REMAINDER, 0.0)
+    head = subprocess.run(
+        ["git", "-C", REPO, "log", "-1", "--format=%h %s"],
+        capture_output=True, text=True, check=True).stdout.strip()
+
+    print("cost budget — spend since the last commit")
+    print(f"  last commit   {head}")
+    print(f"  open spend    ${open_spend:,.2f}")
+    print(f"  soft / hard   ${soft:,.2f} / ${hard:,.2f}")
+    if open_spend > hard:
+        print(f"\n  HARD BREACH — ${open_spend:,.2f} > ${hard:,.2f}. Commit what "
+              f"works, or stop and decompose. Uncommitted work is also "
+              f"unattributable.", file=sys.stderr)
+        return 1
+    if open_spend > soft:
+        print(f"\n  soft breach — ${open_spend:,.2f} > ${soft:,.2f}. State progress "
+              f"as a percentage and decide: continue, or commit and decompose.")
+        return 0
+    print("\n  within budget")
+    return 0
+
+
 def main():
     ap = argparse.ArgumentParser()
     ap.add_argument("--slug", default="-home-worsch-clay-borg")
@@ -489,6 +530,10 @@ def main():
     ap.add_argument("--composition", action="store_true")
     ap.add_argument("--session-shape", action="store_true",
                     help="SH-1..SH-3 (always shown in the default report)")
+    ap.add_argument("--budget", action="store_true",
+                    help="CB-01/CB-02: spend since the last commit, live")
+    ap.add_argument("--soft", type=float, default=10.00)
+    ap.add_argument("--hard", type=float, default=22.00)
     ap.add_argument("--self-test", action="store_true")
     ap.add_argument("--json", action="store_true")
     args = ap.parse_args()
@@ -496,6 +541,9 @@ def main():
     if args.self_test:
         return self_test()
 
+    if args.budget:
+        return budget(args.slug, args.soft, args.hard)
+
     try:
         rep = collect(args.slug, args.pin)
     except Abort as e: