* fix: reuse auth profile in read-only environments

[weekwood]

Summary

• avoid rewriting an existing profile config when the store is initialized for reads
• treat lastUsedTime persistence as best-effort so a valid token remains usable when profile storage is read-only
• add regression coverage for both read paths

Root cause

Configstore received an empty defaults object, which rewrote the config during construction. In addition, ensureAuth() always persisted lastUsedTime. In sandboxed or otherwise read-only environments, either write failed and the CLI surfaced E1005, even though the stored token was valid.

Verification

• bun test — 186 passed, 0 failed
• bun run typecheck
• bun run build
• installed the packed CLI locally and confirmed zentao --version and an authenticated zentao story 326 --format=json both reuse the existing token without another login