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>

src/utils/context.ts

@@ -12,7 +12,12 @@ export const MODEL_CONTEXT_WINDOW_DEFAULT = 200_000
 // Fallback context window for unknown 3P models. Must be large enough that
 // the effective context (this minus output token reservation) stays positive,
 // otherwise auto-compact fires on every message (issue #635).
-export const OPENAI_FALLBACK_CONTEXT_WINDOW = 128_000
+// Override via CLAUDE_CODE_OPENAI_FALLBACK_CONTEXT_WINDOW env var to avoid
+// hardcoding when deploying models not yet in openaiContextWindows.ts.
+export const OPENAI_FALLBACK_CONTEXT_WINDOW = (() => {
+  const v = parseInt(process.env.CLAUDE_CODE_OPENAI_FALLBACK_CONTEXT_WINDOW ?? '', 10)
+  return !isNaN(v) && v > 0 ? v : 128_000
+})()
 
 // Maximum output tokens for compact operations
 export const COMPACT_MAX_OUTPUT_TOKENS = 20_000