fix(run): address round-2 gadfly nits (max(), drop dead soft fallback, decouple doc)

All low-severity follow-ups on the critic-deadline change:
- hardCap uses max(CriticAbsoluteMax, maxRuntime) instead of a nested if (723193a7).
- Drop the now-dead 90s soft-trigger fallback + its bare literal: the sole caller
  passes the resolved MaxRuntime (>0), and Run's unsupervised-run failsafe bounds
  even an impossible 0 (8d377051, 2f86bf58).
- Decouple the kernel doc from a named downstream convar ("a 6h host convar")
  (730c67fc).

Graded false-positive: agent.go BackstopMultiplier validation (handled in the host;
not in this diff), the 24h default "magic number" (matches every withFallbacks
default), and the defer-in-conditional pattern (idiomatic). Kept: the thorough
two-tier comment (this logic regressed once) and the rare-path nested timer.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Jo75sqmeVPgFUWZQBn179X

run/executor.go

@@ -35,11 +35,10 @@ type Defaults struct {
 	// deadline. This ceiling exists ONLY to stop a critic that never advances its
 	// deadline (a broken host handle) from running forever, so it is deliberately
 	// set FAR beyond any realistic backstop (default 24h): the host clamps its own
-	// backstop to a much smaller absolute max (e.g. mort's agents.critic.
-	// absolute_max_seconds = 6h), so the ceiling never pre-empts a healthy
-	// supervised run. Keep it well above the host's absolute max. Never shorter than
-	// the run's MaxRuntime. Non-critic runs ignore it (they keep the literal
-	// MaxRuntime kill).
+	// backstop to a much smaller absolute max (e.g. a 6h host convar), so the ceiling
+	// never pre-empts a healthy supervised run. Keep it well above the host's
+	// absolute max. Never shorter than the run's MaxRuntime. Non-critic runs ignore
+	// it (they keep the literal MaxRuntime kill).
 	CriticAbsoluteMax time.Duration
 }
 
@@ -294,7 +293,7 @@ func (e *Executor) Run(ctx context.Context, ra RunnableAgent, inv tool.Invocatio
 	//     gets a WithTimeout at CriticAbsoluteMax (default 24h) purely as a RUNAWAY
 	//     guard for a critic that never advances its deadline: it is set FAR beyond
 	//     any realistic backstop (the host clamps its own backstop to a much smaller
-	//     absolute max, e.g. mort's 6h convar), so it does NOT pre-empt a healthy
+	//     absolute max, e.g. a 6h host convar), so it does NOT pre-empt a healthy
 	//     supervised run. If the host critic fails to ARM (nil handle), the run is
 	//     unsupervised and we tighten the cap back down to MaxRuntime below.
 	// A NESTED cause-carrying layer (cancelCause) lets a critic kill surface as a
@@ -305,12 +304,9 @@ func (e *Executor) Run(ctx context.Context, ra RunnableAgent, inv tool.Invocatio
 	hardCap := maxRuntime
 	if criticOwns {
 		// Runaway guard only — the critic's own (extendable) deadline-watch is the
-		// normal cap. Never shorter than the nominal budget, in case an operator
-		// sets MaxRuntime above the runaway ceiling (a degenerate config).
-		hardCap = e.cfg.Defaults.CriticAbsoluteMax
-		if hardCap < maxRuntime {
-			hardCap = maxRuntime
-		}
+		// normal cap. max() keeps it from being shorter than the nominal budget if an
+		// operator sets MaxRuntime above the runaway ceiling (a degenerate config).
+		hardCap = max(e.cfg.Defaults.CriticAbsoluteMax, maxRuntime)
 	}
 	timeoutCtx, cancelTimeout := context.WithTimeout(context.WithoutCancel(ctx), hardCap)
 	defer cancelTimeout()