feat: port /goal edit and compaction survival from prevalentWare fork

Adapt two substantive behaviors from the prevalentWare/opencode-goal-plugin
reimplementation into the marker-based plugin, preserving naming/branding:

- /goal edit <objective>: revise the active goal in place, keeping the
  turn/token/time budget and history (from their update_goal_objective tool).
- experimental.session.compacting hook: inject goal objective, status, budget,
  and latest checkpoint into the compaction context so the goal survives a
  session compaction (from their compactionContext injection).

Adds 8 tests (80 total, all passing).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>