Fix Morph handling of OpenCode compaction events

[bhaktatejas922]

Closes #13.

Summary

• Treat OpenCode native compaction as the persisted compaction path in current 1.14.x releases.
• Arm Morph compression from experimental.session.compacting, then compress OpenCode's selected history to one summary in the following experimental.chat.messages.transform call.
• Preserve OpenCode's default compaction prompt anchoring by appending Morph context instead of replacing output.prompt.
• Add logs/toast text and README guidance so users can verify why OpenCode native compaction still appears after the Morph toast, including behavior with 1M context windows.

Tests

• bun test
• bun run build
• bun run typecheck
• bun run ci
• git diff --check