fix(doctor): include custom providers from opencode.json in provider check

The doctor's 'Model override uses unavailable provider' check only
looked at providers from ~/.cache/opencode/models.json (built-in
providers from models.dev). Custom OpenAI-compatible providers defined
in the user's opencode.json (under the 'provider' key) were not
included, causing false-positive warnings.

Now loadAvailableModelsFromCache() also reads provider names from
~/.config/opencode/opencode.json and ~/.config/opencode/opencode.jsonc,
merging them with the cache providers. This eliminates the false
positive while preserving real warnings for truly unknown providers.

7 new tests cover: cache-only, custom-only, merged, deduplicated,
JSONC variant, and malformed config resilience.

Fixes #3199