fix: close codex-acp session config gaps

[vincentkoc]

Summary

• bridge acpx codex set thought_level <value> to codex-acp's current reasoning_effort config id
• refresh codex-facing examples and add a dedicated agents/Codex.md harness doc so the repo stops teaching stale config keys
• align codex session-control CLI tests with the current error payload shape on main

Why

Current codex-acp releases support mode, model, and reasoning_effort, but acpx issue #11 and several codex examples still assume thought_level or older config semantics. This PR closes the highest-value acpx-side compatibility gap without pretending codex-acp has identical config ids to every other adapter.

Related: #11

Test Plan

• pnpm run build:test && node --test --test-name-pattern='codex thought_level aliases to reasoning_effort|set-mode surfaces actionable guidance when agent rejects session/set_mode params|set returns an error when agent rejects unsupported session config params' dist-test/test/cli.test.js
• pnpm run check:docs
• pnpm run check
  Local result: tests completed, but the repo's local test:coverage gate still exited on aggregate coverage thresholds (67.97% branches, 38.34% functions) rather than a codex-specific regression.