fix: sync OpenCode auth state on account rotation

[chubes4]

Summary

• sync refreshed/rotated Anthropic OAuth credentials into OpenCode's live auth state
• preserve the existing auth.json write path as the durable source
• extend the refresh hardening regression guard for live auth sync

Testing

• bash tests/opencode-claude-auth-refresh-hardening.sh
• bash tests/upgrade-opencode-auth-plugin-sync.sh
• bun build runtimes/opencode/plugins/claude-code-auth.ts --target=node --outdir /var/folders/lr/c_cmmt7s0592m4njz99v5yb40000gn/T/opencode/wpca-auth-plugin-check-worktree

AI assistance

• AI assistance: Yes
• Tool(s): GPT-5.5 via OpenCode
• Used for: Compared the standalone OpenCode auth plugin with Kimaki's Anthropic rotation implementation, drafted the fix, and ran targeted verification.