fix: make Codex V2 agent payloads provider-agnostic

[xhwxt]

Summary

• Rewrite Codex V2 agents.* schemas to plaintext whenever the Codex MultiRouter has any third-party or ownership-ambiguous route.
• Apply the rewrite for arbitrary providers, not only official OAuth parents, while leaving pure official routers encrypted.
• Keep reserved collaboration.* schemas and unrelated encrypted fields unchanged.
• Update the design specification and coverage for top-level and Lite tool paths.

Validation

• git diff --check
• In-memory arbitrary-provider routing smoke test
• Rust tests/build not run locally because the Rust toolchain is unavailable in this environment.