fix(opencode-free): replace unhealthy big-pickle bootstrap default

[liugddx]

Problem

A fresh Maka install seeds the anonymous opencode-free connection with big-pickle as its default model. That model currently does not complete within the 15-second connection-test timeout, so the built-in zero-credential provider appears offline even though the OpenCode Zen endpoint and other free models are healthy.

Existing installations retain the same stale default in llm-connections.json, so changing only the provider fallback order would not repair them.

Reproduction

1. Start Maka with a fresh workspace and no provider API keys.
2. Open Settings → Connections.
3. Test the preconfigured OpenCode Free connection.

Expected: the zero-config connection test succeeds and a first chat can use the selected default.

Actual: the request using big-pickle times out and the connection is reported as failed.

Live endpoint check (2026-08-04)

• GET https://opencode.ai/zen/v1/models: 200
• anonymous mimo-v2.5-free: 200
• anonymous nemotron-3-ultra-free: 200
• anonymous laguna-s-2.1-free: 200
• anonymous big-pickle: timed out
• anonymous north-mini-code-free: 500 Router.Unavailable

Proposed fix

• Move the bootstrap default to a currently responsive free model and keep the provider fallback order consistent.
• Migrate only the untouched historical auto-seed (opencode-free + old default/selection), without changing customized connections.
• Add regression coverage for fresh bootstrap and migration behavior.
• Verify the selected default against the live anonymous endpoint.