feat(providers): add OpenAI Codex provider with Responses API support

.env.example

@@ -404,6 +404,65 @@
 # Default: 8085 (same as Gemini CLI, shared)
 # ANTIGRAVITY_OAUTH_PORT=8085
 
+# ------------------------------------------------------------------------------
+# | [CODEX] OpenAI Codex Provider Configuration                                |
+# ------------------------------------------------------------------------------
+#
+# Codex provider uses OAuth authentication with OpenAI's ChatGPT backend API.
+# Credentials are stored in oauth_creds/ directory as codex_oauth_*.json files.
+#
+
+# --- Reasoning Effort ---
+# Controls how much "thinking" the model does before responding.
+# Higher effort = more thorough reasoning but slower responses.
+#
+# Available levels (model-dependent):
+#   - low: Minimal reasoning, fastest responses
+#   - medium: Balanced (default)
+#   - high: More thorough reasoning
+#   - xhigh: Maximum reasoning (gpt-5.2, gpt-5.2-codex, gpt-5.3-codex, gpt-5.1-codex-max only)
+#
+# Can also be controlled per-request via:
+#   1. Model suffix: codex/gpt-5.2:high
+#   2. Request param: "reasoning_effort": "high"
+#
+# CODEX_REASONING_EFFORT=medium
+
+# --- Reasoning Summary ---
+# Controls how reasoning is summarized in responses.
+# Options: auto, concise, detailed, none
+# CODEX_REASONING_SUMMARY=auto
+
+# --- Reasoning Output Format ---
+# How reasoning/thinking is presented in responses.
+# Options:
+#   - think-tags: Wrap in <think>...</think> tags (default, matches other providers)
+#   - raw: Include reasoning as-is
+#   - none: Don't include reasoning in output
+# CODEX_REASONING_COMPAT=think-tags
+
+# --- Identity Override ---
+# When true, injects an override that tells the model to prioritize
+# user-provided system prompts over the required opencode instructions.
+# CODEX_INJECT_IDENTITY_OVERRIDE=true
+
+# --- Instruction Injection ---
+# When true, injects the required opencode system instruction.
+# Only disable if you know what you're doing (API may reject requests).
+# CODEX_INJECT_INSTRUCTION=true
+
+# --- Empty Response Handling ---
+# Number of retry attempts when receiving empty responses.
+# CODEX_EMPTY_RESPONSE_ATTEMPTS=3
+
+# Delay (seconds) between empty response retries.
+# CODEX_EMPTY_RESPONSE_RETRY_DELAY=2
+
+# --- OAuth Configuration ---
+# OAuth callback port for Codex interactive authentication.
+# Default: 8086
+# CODEX_OAUTH_PORT=8086
+
 # ------------------------------------------------------------------------------
 # | [ADVANCED] Debugging / Logging                                              |
 # ------------------------------------------------------------------------------