fix: use prt_ prefix for all part IDs (OpenCode v1.2.25+ compat)

OpenCode v1.2.25+ validates that all part IDs must start with 'prt'.
The chat.message hook was creating parts with IDs like
'supermemory-nudge-xxx' and 'supermemory-context-xxx', causing
ZodError crashes on every session start.

Changes:
- Extract generatePartId/generateMessageId into shared ids.ts
- Use generatePartId() in index.ts for nudge and context parts
- Remove duplicated functions from compaction.ts

Fixes supermemoryai/opencode-supermemory#28
Fixes supermemoryai/opencode-supermemory#29
Fixes supermemoryai/opencode-supermemory#32