st_navigate/st_travel: return ETA in seconds + explicit wait(N) nudge — ISO-timestamp arrivals cause under-waiting

[mabry1985]

Problem

Split from protoLabsAI/protoAgent#1702 (the plugin half). st_navigate/st_travel return the arrival as an ISO timestamp, so the agent must diff it against current_time itself — in practice it lowballs, waits ~30s of a ~200s transit, wakes early, and re-waits. On hosts without the core wait-dedup fix each short wait scheduled ANOTHER stacked wake (see the core issue); even with dedup, under-waiting burns iteration budget for nothing.

st_ship already computes arriving in Ns via _eta_seconds — the nav tools should too.

Proposed fix

• st_navigate/st_travel (and any tool returning an arrival/cooldown) also return the remaining time in seconds (arriving in Ns), reusing _eta_seconds.
• Append an explicit nudge to the tool output — e.g. wait(<N>, "...") — so the agent waits the full transit in one call instead of polling.

Acceptance

• Nav/travel tool outputs include integer seconds remaining, not just the ISO arrival
• Output includes an explicit wait-this-long nudge
• Autopilot/watchdog guidance mentions the one-wait-per-transit pattern

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