feat(pi): add NaN.Builders custom provider

[barbatdev]

Goal

Configure NaN.Builders as a custom Pi provider using the documented ~/.pi/agent/models.json format.

Scope

• Add the nan provider with https://api.nan.builders/v1.
• Use the openai-completions adapter.
• Add the qwen3.6 model.
• Resolve the API key from the existing protected NaN key file at request time.
• Integrate pi/models.json with the existing dotfiles installer convention.
• Validate JSON and run one bounded fake Pi request against loopback only.

Non-goals

• API-key rotation helper.
• OpenCode wrapper or configuration mutation.
• Zsh command additions.
• Real credentials or real NaN requests.

Delivery constraints

Delegated-direct, provider-only, no SDD, RDD disabled/unmanaged, no commit or PR publication in this work unit.