Honor workspace-write sandbox config on every turn

[Greg-Moskalenko]

What changed

• preserve explicit sandbox_workspace_write.network_access = true when codex-acp builds each workspace-write turn policy
• merge explicit absolute sandbox_workspace_write.writable_roots into that same per-turn policy, with exact deduplication
• preserve on-request approvals, ACP additional directories, and the existing temporary-directory flags
• leave read-only and full-access modes unchanged; ignore malformed, non-string, and non-absolute configured roots

Why

codex-acp passes a mode-specific sandbox policy on every turn. The default Agent mode hardcodes networkAccess: false and an empty root list, so it overrides a caller's valid session-level workspace-write network and root configuration. A client can therefore create a correctly configured thread but receive turns that lack both its requested network access and isolated writable roots.

This change carries only the explicit workspace-write settings into the per-turn workspace-write policy. It does not select danger-full-access, widen another mode, or alter the approval policy.

Validation

• npm run typecheck
• npm test (38 files passed, 6 skipped; 349 tests passed, 28 skipped)
• npm run bundle:all (Linux x64/arm64, macOS x64/arm64, Windows x64/arm64)
• independent read-only review: no findings