[FEATURE]:Add pre-compaction hook for session save/export #14164

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Describe the enhancement you want to request

Description:

When a long conversation triggers context compaction, the full conversation history is permanently lost -- only a summary survives. There's currently no way to automatically save or export the session before this happens.

Related issues: #9387 (session export CLI), #7579 (plan persistence across compaction). This request differs in that it proposes an automatic local save triggered before compaction runs, rather than a manual export command.

Use case:

During extended coding sessions, compaction can trigger unexpectedly. The /share command exists but requires manual intervention (and cloud authentication). Users working in air-gapped or corporate environments may not have access to /share at all.

Proposed solution:

Add a pre-compaction lifecycle hook that fires before the context is compacted. This could support:

1. Auto-export to a local file -- save the full conversation (messages + tool outputs) to a local file (e.g., .opencode/sessions/) before compaction runs
2. Plugin hook -- expose a beforeCompaction event in the plugin system so users can implement custom save logic
3. Config option -- a simple config flag like compaction.save_before: true that writes a local snapshot automatically

Example config:

{
  "compaction": {
    "auto": true,
    "save_before": true,
    "save_path": ".opencode/sessions/"
  }
}

Why this matters:

• Long sessions accumulate valuable context about decisions, debugging steps, and rationale that's useful to revisit later
• Compaction summaries lose nuance and exact details
• A local-first approach works in all environments without requiring cloud auth