Add codex-shim doctor command

[luckeyfaraday]

Summary

• Adds a read-only codex-shim doctor command that inspects and reports on the full shim setup without modifying anything
• Accepts an optional provider argument to narrow checks: codex-shim doctor openrouter, codex-shim doctor minimax
• Exit 0 if all checks pass or only warnings; exit 1 on hard failures

What it checks

Check	Details
imports	codex_shim loads correctly, reports version
codex-cli	codex binary on PATH
port N	available, occupied by another process, or shim already running
settings	~/.codex-shim/models.json exists, parses, lists slugs + providers; flags missing API keys
codex.toml	~/.codex/config.toml managed status, active model, which keys would be preserved on enable/disable
auth.json	gpt-5.5 passthrough availability
per-provider	settings file exists, parses, models have valid provider + base_url + API key

Sample output

  ok    imports     codex_shim 0.1.0 loaded
  ok    codex-cli   /usr/local/bin/codex
  ok    port 8765   available
  warn  settings    ~/.codex-shim/models.json not found — create it or use --settings
  warn  codex.toml  ~/.codex/config.toml exists but shim not installed — run codex-shim enable
  ok    auth.json   gpt-5.5 passthrough available

  OpenRouter
  warn  settings    ~/.codex-shim/openrouter-models.json not found — run: codex-shim setup openrouter

  MiniMax Token Plan
  warn  settings    ~/.codex-shim/minimax-models.json not found — run: codex-shim setup minimax

Test plan

• codex-shim doctor runs without errors on a clean install
• codex-shim doctor openrouter and codex-shim doctor minimax run correctly
• All 36 existing non-async tests pass
• API keys are never printed — only present / MISSING
• No files are modified — strictly read-only

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