feat(runtime): C3 tunable reaper threshold + heartbeat on every verb dispatch

Smoke run 3 showed agents reaped at the 3-min stale-claim window
while they were actively retrying rejected verbs. Two causes:

1. The reaper threshold was hardcoded at 180s via claim_stale_seconds.
   LLM inference + retry loops routinely take longer than that between
   verb-successes. Added settings.stale_claim_reap_seconds (default
   600s); override via ROBOCO_STALE_CLAIM_REAP_SECONDS env var.
   claim_stale_seconds (spawn-filter cutoff) is unchanged at 180s.

2. last_heartbeat_at only refreshed on verb SUCCESS. A verb stuck
   in a rejection loop (e.g. tracing_gap missing journal:decision)
   showed no heartbeat updates even though the agent was alive.
   Added a best-effort heartbeat refresh inside _emit_rejection so
   EVERY verb dispatch — success or rejection — counts as activity.

Heartbeat approach: option (b) — touch inside _emit_rejection (single
centralized rejection path). Requires no middleware layer, no HTTP body
parsing, and no new files. The _touch guard for task_id=None means
agent-level rejections (no task context) are a safe no-op.

Net effect: agents stop being reaped mid-retry. Genuinely-stuck
containers (no verb dispatch at all) still reap normally at 600s.

Spec ref: Wave C Task C3.

roboco/config.py

@@ -326,6 +326,22 @@ class Settings(BaseSettings):
         ge=60,
         description="Claim heartbeat staleness threshold (seconds)",
     )
+    # Wave C3 (2026-05-12). Reaper window for stale-claim detection.
+    # Smoke run 3 reaped agents at ~180s while they were actively
+    # retrying — LLM inference + retry loops routinely exceed 3 min
+    # between verb successes. 600s is large enough to accommodate that
+    # without letting a genuinely-stuck container linger.
+    # Distinct from claim_stale_seconds (which drives trigger_filter
+    # spawn queueing); keeping them separate avoids a window where a
+    # higher reap threshold would also delay spawn-queue decisions.
+    stale_claim_reap_seconds: int = Field(
+        default=600,
+        ge=60,
+        description=(
+            "Reaper-only stale claim threshold (seconds); "
+            "override via ROBOCO_STALE_CLAIM_REAP_SECONDS"
+        ),
+    )
     spawn_cooldown_seconds: int = Field(
         default=60,
         ge=1,