fix: align opencode v2 plugin contract

[Nomadcxx]

Summary

• Follow up feat: add OpenCode v2 plugin support (dual V1/V2 entrypoint) #124 by writing the proxy URL through V2 provider settings.
• Register complete V2 tool objects and leave session tool records to the host.
• Clear stale credentials and dispose V2 registrations and MCP clients.

Verification

• npm run build
• Unit: 591 pass, 0 fail
• Integration: 43 pass, 1 skip, 0 fail
• npm pack --dry-run
• Host runtime harness: provider routing, tool schema, credential scope, and cleanup