Prepare required workspace through native connection pool - #623

Required workspace preparation used the native Turn discovery helper, whose short timeout returns an empty inventory after a failed attempt. Preparation now discovers and validates the bridge through the existing Session-scoped connection pool, with the native connection timeout and the unchanged ACP/Core deadlines. Turn discovery and optional MCP lazy connections retain their existing behavior.

The change also records safe native preparation phase, duration and outcome fields without logging native errors or credentials.

Validation: native pool/SDK lifecycle and prompt regressions pass, including readiness beyond the shortened Turn threshold, configured timeout, interrupted initialization, configuration cleanup and selected-server reconnection. Go Mcode tests/vet and name, documentation and CI checks pass. The baseline fails the new startup-budget regression. The cached native build and matching 500 ms File-service prepare-only comparison pass: ready at 74.621 s, native MCP connect 15.201 s, zero model requests, and confirmed owner shutdown. The baseline failed Session creation at 15.236 s. File calls remain 192 in both cases; this is a correctness fix, not a latency optimization. Live acceptance remains pending.

Native service shutdown joins initialization and mutation cleanup; final process containment remains owned by the Executor/View. This corrects startup discovery semantics and does not establish the cause of the earlier live failure.