fix(acp): share acp-next session state

[nexxeln]

Summary

• route acp-next new/load session state through the Phase 2 ACPNextSession service
• route directory snapshots through Directory.Service with an SDK-backed loader for this flagged process
• avoid stale MCP registration skips by deduping per session and MCP config identity

Validation

• PATH="$HOME/.bun/bin:$PATH" bun test test/acp-next test/cli/acp-next
• /Users/nxl/.volta/bin/node node_modules/@typescript/native-preview/bin/tsgo.js --noEmit
• pre-push hook: bun turbo typecheck