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

ui/src/components/cliproxy/provider-editor/use-provider-editor.ts

@@ -12,7 +12,7 @@ import type { ProviderCatalog } from '../provider-model-selector';
 import {
   applyExtendedContextPreferenceToAnthropicModels,
   hasAnthropicExtendedContextEnabled,
-  isAnthropicModelEnvKey,
+  isExtendedContextModelEnvKey,
 } from '@/lib/extended-context-utils';
 import { supportsExtendedContext } from '@/lib/model-catalogs';
 import { isValidProvider } from '@/lib/provider-config';
@@ -104,7 +104,7 @@ export function useProviderEditor(
   const updateEnvValue = useCallback(
     (key: string, value: string) => {
       const newEnv = { ...(currentSettings?.env || {}), [key]: value };
-      const envWithIntent = isAnthropicModelEnvKey(key)
+      const envWithIntent = isExtendedContextModelEnvKey(key)
         ? applySavedLongContextIntent(newEnv, extendedContextEnabled)
         : newEnv;
       delete envWithIntent['CCS_EXTENDED_CONTEXT'];
@@ -132,7 +132,7 @@ export function useProviderEditor(
   const updateEnvValues = useCallback(
     (updates: Record<string, string>) => {
       const newEnv = { ...(currentSettings?.env || {}), ...updates };
-      const touchesAnthropicModel = Object.keys(updates).some(isAnthropicModelEnvKey);
+      const touchesAnthropicModel = Object.keys(updates).some(isExtendedContextModelEnvKey);
       const envWithIntent = touchesAnthropicModel
         ? applySavedLongContextIntent(newEnv, extendedContextEnabled)
         : newEnv;