[P2] Complete the machine-readable Codex status and thread-discovery contract #39

Summary: Make status and list-threads dependable for automation.
Why: permission failures collapse into absence, version equality is not schema compatibility, version probing can hang, nextCursor cannot be supplied, limits/unknown args are weakly handled, and terminal controls survive in fields.
Acceptance criteria: structured failure states and consistent exits; bounded version probe; endpoint reachability distinct from Desktop ownership and schema compatibility; RPC shape validation; cursor input/output and page bounds; reject unknown args; harden text fields; test failures, pagination, timeouts, Windows refusal, and compatibility fixtures.

Parent: #24
Source: GPT-6 Pro post-land review of #27/#28 — https://chatgpt.com/c/6aa89f23-6920-83ea-b98a-a1f155a1ae3f