fix: make OpenAI fallback context window configurable + support external model lookup (#861)

* fix: make OpenAI fallback context window configurable and support external lookup table

Unknown OpenAI-compatible models fell back to a hardcoded 128k constant,
causing auto-compact to fire prematurely on models with larger windows
(issue #635 follow-up). Two escape hatches are added without touching the
built-in table:

- CLAUDE_CODE_OPENAI_FALLBACK_CONTEXT_WINDOW (number): overrides the 128k
  default for all unknown models.
- CLAUDE_CODE_OPENAI_CONTEXT_WINDOWS (JSON object): per-model overrides that
  take precedence over the built-in OPENAI_CONTEXT_WINDOWS table; supports
  the same provider-qualified and prefix-matching lookup as the built-in path.
- CLAUDE_CODE_OPENAI_MAX_OUTPUT_TOKENS (JSON object): same pattern for output
  token limits.

This lets operators deploy new or private models without patching
openaiContextWindows.ts on every model release.

* docs: add new OpenAI context window env vars to .env.example

Document CLAUDE_CODE_OPENAI_FALLBACK_CONTEXT_WINDOW,
CLAUDE_CODE_OPENAI_CONTEXT_WINDOWS, and
CLAUDE_CODE_OPENAI_MAX_OUTPUT_TOKENS with usage examples.

Addresses reviewer feedback on PR #861.

---------

Co-authored-by: opencode <dev@example.com>

.env.example

@@ -149,6 +149,23 @@ ANTHROPIC_API_KEY=sk-ant-your-key-here
 # Use a custom OpenAI-compatible endpoint (optional — defaults to api.openai.com)
 # OPENAI_BASE_URL=https://api.openai.com/v1
 
+# Fallback context window size (tokens) when the model is not found in the
+# built-in table (default: 128000). Increase this for models with larger
+# context windows (e.g. 200000 for Claude-sized contexts).
+# CLAUDE_CODE_OPENAI_FALLBACK_CONTEXT_WINDOW=128000
+
+# Per-model context window overrides as a JSON object.
+# Takes precedence over the built-in table, so you can register new or
+# custom models without patching source.
+# Example: CLAUDE_CODE_OPENAI_CONTEXT_WINDOWS={"my-corp/llm-v3":262144,"gpt-4o-mini":128000}
+# CLAUDE_CODE_OPENAI_CONTEXT_WINDOWS=
+
+# Per-model maximum output token overrides as a JSON object.
+# Use this alongside CLAUDE_CODE_OPENAI_CONTEXT_WINDOWS when your model
+# supports a different output limit than what the built-in table specifies.
+# Example: CLAUDE_CODE_OPENAI_MAX_OUTPUT_TOKENS={"my-corp/llm-v3":8192}
+# CLAUDE_CODE_OPENAI_MAX_OUTPUT_TOKENS=
+
 
 # -----------------------------------------------------------------------------
 # Option 3: Google Gemini