feat: raise default duration to 15 min, rewrite README

Default maxDurationMs changed from 5 min to 15 min so the turn limit is
the binding brake at typical LLM latency (30-90 s/turn). At 5 min, users
hitting a goal with a slow model or large context would exhaust the timer
well before the 10-turn limit they expected.

README rewritten: clearer structure, limits table with effective-turn-count
and token-budget notes, per-goal flags table, updated config examples.
Version bumped to 0.1.5.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>