OpenCode pins sessions, keeps costs visible

This is the right shape for coding agents: one durable conversation per task, with compaction turning long-lived chats from liability into workflow.

• –Pinning a session per PR reduces context thrash and makes it easier to pick up the same branch days later.
• –Compaction is the key enabler here; without it, long-running agent sessions would turn into brittle, expensive context dumps.
• –Visible cost tracking matters because it turns agent usage from “feels fast” into something teams can budget and compare.
• –The experimental flag suggests the session UX is still evolving, but the direction is clear: persistent, task-scoped agent work instead of throwaway chats.