test: guard synthetic part id compatibility

[walioo]

Summary

• extract synthetic Supermemory part ID creation into a small helper with regression tests
• add a bun test release gate so future publishes cannot skip this compatibility check
• keep the existing prt_supermemory-* behavior but make it explicitly verified in CI

Why

OpenCode 1.2.25+ enforces prt-prefixed part IDs. The repository source already contains the prefix fix, but the published 2.0.5 package still reproduced runtime failures with synthetic IDs like supermemory-context-* in my environment. This PR adds test and release protection so the compatibility fix remains enforced and future publishes cannot regress silently.

Verification

• bun test
• bun run typecheck
• bun run build
• local OpenCode 1.3.2 repro with patched installed plugin no longer emitted Invalid string: must start with \"prt\"