Stop OpenCode refresh from leaking serve processes

OpenCode can leave the long-lived serve child in the spawned process group after the wrapper exits. Provider refresh owns only a scoped inventory probe, so cleanup targets that local process group when the refresh scope closes.

Constraint: Keep active OpenCode thread sessions scoped to their own session lifecycle.
Rejected: Disable provider refresh | would hide model and auth state changes.
Confidence: high
Scope-risk: narrow
Tested: bun run test src/provider/Layers/OpenCodeProvider.test.ts src/provider/Layers/OpenCodeAdapter.test.ts
Tested: bun fmt
Tested: bun lint
Tested: bun typecheck
Tested: bun run build:desktop