Skip context-1m beta header for excluded models

[constkolesnyak]

Problem

Some Claude OAuth subscriptions reject the context-1m-2025-08-07 beta for specific models. Example: a session on claude-sonnet-4-6 returns

400 long context beta not yet available for this subscription

while the same subscription accepts the beta on claude-opus-4-8. The current global agent.context_1m: bool is all-or-nothing — turning it off to fix Sonnet kills the 1M context on Opus too. Affected sessions break entirely with context_1m: true until manually downgraded.

Fix

Add AgentConfig.context_1m_excluded_models: list[str] — case-insensitive substring match against the resolved model name. When context_1m=True, the beta header is sent for every model whose name does not match any entry.

agent:
  context_1m: true
  context_1m_excluded_models:
    - claude-sonnet-4-6

New helper AgentConfig.context_1m_enabled_for(model: str | None) -> bool drives both:

1. Beta header decision at session start (engine.py: betas = ["context-1m-2025-08-07"] if context_1m_enabled_for(model) else [])
2. max_context value used for context-bar usage metadata (so excluded sessions report the right 200k ceiling, not the 1M one)

Empty list keeps the existing all-or-nothing behaviour — no behaviour change for users who don't set the new field.

Tests

Existing test suite stays green: 1151 passed, 2 skipped (excluding pre-existing tests/test_cron.py::TestMaybeRotateContext rotate_at failures unrelated to this change — present on main too).

The helper is a pure function on AgentConfig; substring match + the context_1m global guard cover the matrix.

Files

• nerve/config.py — context_1m_excluded_models field + context_1m_enabled_for(model) helper
• nerve/agent/engine.py — use the helper at the beta-header decision and at the max_context site

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