Session compaction API returns "not available yet" — compact-now skill non-functional #38085

Bug Description

The session compaction API POST /api/session/{sessionID}/compact returns:

{"_tag":"ServiceUnavailableError","message":"Session compact is not available yet","service":"session.compact"}

This causes the compact-now skill to return true without performing any actual session compaction.

Reproduction

1. Start opencode serve: opencode serve --port 4096
2. Call POST /api/session/{sessionID}/compact on any session
3. Response: {"_tag":"ServiceUnavailableError","message":"Session compact is not available yet"}

Expected Behavior

The API should either perform session compaction and return success, or return a proper error if genuinely unavailable.

Environment

• OpenCode version: 1.18.4
• Server running on port 4096
• Models: big-pickle (opencode/big-pickle), MiniMax-M2.7-highspeed, deepseek-v4-flash-free

Additional Context

The internal agent=compaction engine runs (visible in server logs) and makes AI calls to generate summaries, but time_compacting is never set in the database — compaction never completes.

Log excerpt showing compaction failures:

level=ERROR message="stream error" agent=compaction error.error="AI_APICallError: Token Plan usage limit reached"
level=ERROR message="stream error" agent=compaction error.error="AI_APICallError: Unauthorized"
level=ERROR message="stream error" agent=compaction error.error="This model's maximum context length is 196608 tokens. However, your messages resulted in 280703 tokens."

Compaction appears to target a dedicated session rather than the session being compacted, and the summarization result is never persisted back to the database (time_compacting stays NULL for all sessions).