feat(engine): manual /compact command and configurable compaction threshold

[EngineerProjects]

Summary

Context compaction currently runs automatically when the context window
approaches the model limit. Add a /compact slash command so users can
trigger it manually, and expose the threshold as a configurable setting.

Scope

• /compact slash command in the TUI — triggers compaction immediately
• nexus config set compaction.threshold 0.8 — set the auto-trigger fraction
• nexus config set compaction.enabled false — disable auto-compaction
• Compaction summary displayed in the chat after /compact completes

Acceptance criteria

• /compact registered in the commands palette and slash-command handler
• Manual compaction produces the same summary message as auto-compaction
• compaction.threshold config key accepted and persisted
• compaction.enabled config key accepted and persisted
• Docs updated in docs/sdk.md under "Long-context compaction"