fix(model-benchmark): don't drop the claude provider on macOS subscription installs

[jbetala7]

Problem

ClaudeAdapter.available() (test/helpers/providers/claude.ts) treats Claude auth as present only when ~/.claude/.credentials.json exists or ANTHROPIC_API_KEY is set:

const credsPath = path.join(os.homedir(), '.claude', '.credentials.json');
const hasCreds = fs.existsSync(credsPath);
const hasKey = !!process.env.ANTHROPIC_API_KEY;
if (!hasCreds && !hasKey) {
  return { ok: false, reason: 'No Claude auth found. ...' };
}

On the default macOS Claude Code subscription install, ~/.claude/.credentials.json is absent — the credential lives in the login Keychain — yet the subscription session authenticates claude -p fine. So the file-existence check is a proxy that misses a valid auth state, and available() reports NOT READY for a provider whose run() path (claude -p --output-format json) would actually succeed. Any caller gated on this available() (model-benchmark, and the LLM-judge path) silently drops the claude provider for that whole population.

On main (cab774c)

$ gstack-model-benchmark --dry-run --prompt "hi"
  claude: NOT READY — No Claude auth found. ...

on a machine where ~/.claude/.credentials.json is absent, claude --version resolves, and claude -p works.

Root cause

An absent ~/.claude/.credentials.json on macOS is not evidence of missing auth — it is the expected state for a Keychain-backed subscription install. available() rejecting an auth state that run() can actually use is an internal inconsistency, not a user-config gap.

Fix

• Keep the positive signals (creds file or ANTHROPIC_API_KEY) as a fast yes.
• On darwin, be optimistic when the binary resolves and defer the real auth decision to run(), which already classifies a genuinely logged-out state as an auth error (/unauthorized|auth|login/i). So a logged-out mac still surfaces a clear auth error from run() instead of being silently dropped at available().
• Non-macOS keeps the strict file/key check, where ~/.claude/.credentials.json is the actual credential store.

Testing

Adds test/claude-adapter-available.test.ts (resolver satisfied with a real binary via GSTACK_CLAUDE_BIN, os.homedir pointed at an empty dir, process.platform overridden — no live CLI):

• macOS, no creds file, no key → available (fails on main, passes here)
• non-macOS, no creds file, no key → not available, with the auth reason
• ANTHROPIC_API_KEY set → available on any platform
• present .credentials.json → available on non-macOS
• unresolvable binary → not available, before the auth sniff

bun test test/claude-adapter-available.test.ts          # 5 pass
bun test test/benchmark-runner.test.ts test/benchmark-cli.test.ts test/claude-adapter-available.test.ts  # 29 pass

Fixes #1890

🤖 Generated with Claude Code

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