fix(extended-context): never write [1m] into the Fable tier key

Claude Code resolves ANTHROPIC_DEFAULT_FABLE_MODEL through a resolver that
strips the [1m] suffix before use, unlike the opus/sonnet resolvers which
pass their env value through untouched. The stripped value is then treated
as an env-supplied default and held to the standard 200k window instead of
the model's native 1M, so writing [1m] into that key costs the long context
window rather than granting it.

Measured against a Claude Code 2.1.259 session (`/context` on a live proxy
endpoint):

  ANTHROPIC_MODEL=claude-fable-5-1[1m]               -> 1m window
  ANTHROPIC_MODEL=claude-fable-5-1                   -> 200k window
  ANTHROPIC_DEFAULT_FABLE_MODEL=claude-fable-5-1[1m] -> 200k window

The fix is key-scoped rather than model-scoped, since the suffix is what
grants 1M on every other key. Adds envKeyAcceptsExtendedContextSuffix() as
the single place recording which keys reject the suffix, enforces it as a
floor inside applyExtendedContextPreferenceToAnthropicModels() so no caller
can bypass it, and strips a previously saved suffix from that key on the
next launch through the auto path.

Also extends the preference to CLAUDE_CODE_SUBAGENT_MODEL. Subagent windows
come from the same resolver as the main loop (the Explore inheritCap bounds
the model tier, not the context window), so a bare subagent model id sits at
200k while the suffixed form gets 1M. The new key list is kept separate from
ANTHROPIC_MODEL_ENV_KEYS, which also drives routing, model-id normalization
and profile validation.

Claude-Session: https://claude.ai/code/session_01NPkafQjVf4pSwPBwBistGk

tests/unit/shared/extended-context-utils.test.ts

@@ -2,7 +2,12 @@ import { describe, expect, it } from 'bun:test';
 
 import {
   ANTHROPIC_MODEL_ENV_KEYS,
+  EXTENDED_CONTEXT_MODEL_ENV_KEYS,
+  applyExtendedContextPreferenceToAnthropicModels,
+  envKeyAcceptsExtendedContextSuffix,
+  hasAnthropicExtendedContextEnabled,
   isAnthropicModelEnvKey,
+  isExtendedContextModelEnvKey,
 } from '../../../src/shared/extended-context-utils';
 
 describe('Anthropic model env keys', () => {
@@ -22,3 +27,84 @@ describe('Anthropic model env keys', () => {
     expect(isAnthropicModelEnvKey('ANTHROPIC_BASE_URL')).toBe(false);
   });
 });
+
+describe('extended-context model env keys', () => {
+  it('covers the Anthropic tiers plus the subagent model key', () => {
+    expect([...EXTENDED_CONTEXT_MODEL_ENV_KEYS]).toEqual([
+      ...ANTHROPIC_MODEL_ENV_KEYS,
+      'CLAUDE_CODE_SUBAGENT_MODEL',
+    ]);
+    expect(isExtendedContextModelEnvKey('CLAUDE_CODE_SUBAGENT_MODEL')).toBe(true);
+    expect(isAnthropicModelEnvKey('CLAUDE_CODE_SUBAGENT_MODEL')).toBe(false);
+  });
+
+  it('marks the fable tier key as suffix-stripping', () => {
+    expect(envKeyAcceptsExtendedContextSuffix('ANTHROPIC_DEFAULT_FABLE_MODEL')).toBe(false);
+    expect(envKeyAcceptsExtendedContextSuffix('ANTHROPIC_MODEL')).toBe(true);
+    expect(envKeyAcceptsExtendedContextSuffix('ANTHROPIC_DEFAULT_OPUS_MODEL')).toBe(true);
+    expect(envKeyAcceptsExtendedContextSuffix('CLAUDE_CODE_SUBAGENT_MODEL')).toBe(true);
+  });
+});
+
+describe('applyExtendedContextPreferenceToAnthropicModels', () => {
+  it('never writes [1m] into the fable tier key and strips a saved one', () => {
+    const env = applyExtendedContextPreferenceToAnthropicModels(
+      {
+        ANTHROPIC_MODEL: 'claude-fable-5-1',
+        ANTHROPIC_DEFAULT_OPUS_MODEL: 'claude-opus-5',
+        ANTHROPIC_DEFAULT_FABLE_MODEL: 'claude-fable-5-1[1m]',
+        CLAUDE_CODE_SUBAGENT_MODEL: 'claude-fable-5-1',
+      },
+      true
+    );
+
+    expect(env).toEqual({
+      ANTHROPIC_MODEL: 'claude-fable-5-1[1m]',
+      ANTHROPIC_DEFAULT_OPUS_MODEL: 'claude-opus-5[1m]',
+      ANTHROPIC_DEFAULT_FABLE_MODEL: 'claude-fable-5-1',
+      CLAUDE_CODE_SUBAGENT_MODEL: 'claude-fable-5-1[1m]',
+    });
+  });
+
+  it('strips every managed key when the preference is off', () => {
+    const env = applyExtendedContextPreferenceToAnthropicModels(
+      {
+        ANTHROPIC_MODEL: 'claude-opus-5[1m]',
+        ANTHROPIC_DEFAULT_FABLE_MODEL: 'claude-fable-5-1[1m]',
+        CLAUDE_CODE_SUBAGENT_MODEL: 'claude-fable-5-1[1m]',
+      },
+      false
+    );
+
+    expect(env).toEqual({
+      ANTHROPIC_MODEL: 'claude-opus-5',
+      ANTHROPIC_DEFAULT_FABLE_MODEL: 'claude-fable-5-1',
+      CLAUDE_CODE_SUBAGENT_MODEL: 'claude-fable-5-1',
+    });
+  });
+
+  it('honors a caller compatibility predicate on top of the key guard', () => {
+    const env = applyExtendedContextPreferenceToAnthropicModels(
+      {
+        ANTHROPIC_MODEL: 'claude-opus-5',
+        ANTHROPIC_DEFAULT_HAIKU_MODEL: 'claude-haiku-4-5-20251001',
+      },
+      true,
+      { supportsExtendedContext: (modelId) => !modelId.startsWith('claude-haiku-') }
+    );
+
+    expect(env).toEqual({
+      ANTHROPIC_MODEL: 'claude-opus-5[1m]',
+      ANTHROPIC_DEFAULT_HAIKU_MODEL: 'claude-haiku-4-5-20251001',
+    });
+  });
+
+  it('reads saved intent from the subagent key too', () => {
+    expect(
+      hasAnthropicExtendedContextEnabled({ CLAUDE_CODE_SUBAGENT_MODEL: 'claude-fable-5-1[1m]' })
+    ).toBe(true);
+    expect(
+      hasAnthropicExtendedContextEnabled({ ANTHROPIC_DEFAULT_FABLE_MODEL: 'claude-fable-5-1' })
+    ).toBe(false);
+  });
+});