fix(design): create OpenAI key file owner-only to close write-then-chmod race

[bunlongheng]

What

saveApiKey() in design/src/auth.ts persists the OpenAI API key to ~/.gstack/openai.json by writing at the process default umask and only tightening permissions afterwards:

fs.writeFileSync(configPath(), JSON.stringify({ api_key: key }, null, 2));
fs.chmodSync(configPath(), 0o600);

On a typical umask (022) the file is created 0644 (group/world-readable). Between the writeFileSync and the chmodSync there is a window where the file containing the OpenAI API key is readable by any other local user. This is CWE-377 (insecure file creation) / CWE-367 (TOCTOU) — low severity, only exploitable by a co-located local user on a shared/multi-user host, but it's a real key-disclosure window and a cheap thing to close.

Fix

Create the file owner-only up front by passing { mode: 0o600 } to writeFileSync, so it is never briefly world-readable. The trailing chmodSync is kept as a backstop that still tightens a pre-existing loose file.

fs.writeFileSync(configPath(), JSON.stringify({ api_key: key }, null, 2), { mode: 0o600 });
fs.chmodSync(configPath(), 0o600);

This matches the convention already used for session files in design/src/session.ts ({ mode: 0o600 } on create, landed in #859). #859 hardened the session files but the actual API key file kept the write-then-chmod pattern — this brings it in line.

Test

Adds a saveApiKey regression test that writes the key under a deliberately permissive umask (0o000) and asserts the resulting file is exactly 0600 with no group/other bits.

$ bun test design/test/auth.test.ts
 7 pass
 0 fail

Scope

Two files, design/src/auth.ts (+3/-1) and design/test/auth.test.ts (+21). No behavior change beyond the file-creation mode.

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