[FEATURE]: Native /goal command foundation #32925

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Add a native /goal command foundation for persistent, evidence-driven engineering goals.

This would let OpenCode create, inspect, pause, resume, clear, budget, log, and review history for workspace-local goals using native state instead of prompt-only custom commands.

Benefits:

• persistent goal state under the workspace
• explicit state machine and checkpoints
• evidence-backed completion instead of model claims
• native /goal command routing
• foundation for future execution-loop work

A draft implementation PR is available for design review and may need to be split before merge.