fix: harden OpenCode Anthropic OAuth refresh

[chubes4]

Summary

• serialize Anthropic OAuth refreshes with a shared lock
• re-read OpenCode auth after lock acquisition so losing workers reuse a freshly rotated token
• retry remembered OAuth accounts on invalid refresh-token failures and report redacted diagnostics

Testing

• tests/opencode-claude-auth-refresh-hardening.sh
• tests/opencode-local-plugin-path.sh
• bun build runtimes/opencode/plugins/claude-code-auth.ts --outfile /var/folders/lr/c_cmmt7s0592m4njz99v5yb40000gn/T/opencode/claude-code-auth-check.js --target=node

AI assistance

• AI assistance: Yes
• Tool(s): gpt-5.5 via OpenCode
• Used for: Diagnosed the OpenCode Anthropic OAuth refresh-token race, implemented the plugin hardening, and added focused regression coverage.