Fix invalid synthetic part IDs

[walioo]

Title: Fix synthetic chat.message part IDs to use valid prt_ format

Body:

Summary

This fixes a compatibility issue with recent OpenCode versions where the plugin injects synthetic chat.message parts with IDs like supermemory-nudge-* and supermemory-context-*.

OpenCode validates part IDs and expects them to start with prt_. Because of that, any prompt could fail before sending with:

ZodError: Invalid string: must start with "prt"

What changed

• Exported generatePartId() from src/services/compaction.ts
• Reused generatePartId() for the synthetic nudge part in src/index.ts
• Reused generatePartId() for the synthetic context part in src/index.ts

Why this approach

The plugin already has a canonical helper for generating valid part IDs. Reusing that helper keeps ID generation consistent across hook-injected and storage-injected parts, and avoids duplicating format knowledge in multiple places.

Verification

• bun run typecheck
• bun run build

Repro

Before this patch, enabling opencode-supermemory could cause a prompt like "你好" to fail during message creation in OpenCode with a schema validation error on parts[].id.

After this patch, the same prompt succeeds and the plugin continues to inject memory context normally.

[gragragrab]

Checked and it works.