feat: provider-boundary v1 with shared tool interception

[Nomadcxx]

Summary

• add a provider-boundary abstraction with legacy/v1 mode selection via CURSOR_ACP_PROVIDER_BOUNDARY
• wire plugin chat/provider/tool-loop decisions through boundary helpers (tool option resolution, provider matching, model normalization, defaults)
• extract shared runtime tool_call handling to remove duplicated interception/update/result logic across Bun and Node streaming paths
• keep rollback safety by preserving legacy mode and parity behavior while enabling v1 rollout
• add boundary-focused unit tests and integration coverage for provider-prefixed model normalization (cursor-acp/)
• document boundary rollout env var in README

Key Files

• src/provider/boundary.ts
• src/provider/runtime-interception.ts
• src/plugin.ts
• tests/unit/provider-boundary.test.ts
• tests/integration/opencode-loop.integration.test.ts
• README.md

Validation

• bun run test:ci:unit
• bun run test:ci:integration

Both suites pass locally in this environment.