feat(codex): support OpenCode ChatGPT OAuth + fix MiniMax percentage-based quota (closes #78)

OpenCode (opencode-codex) ChatGPT login now feeds the existing Codex
provider. Detection falls back to ~/.local/share/opencode/auth.json
(honoring OPENCODE_HOME/XDG_DATA_HOME), tokens are refreshed and written
back in OpenCode's native format (ms expiry), and it is configurable via
`onwatch setup`, install.sh, OPENCODE_ENABLED, and the dashboard.

MiniMax: adapt to the API moving from per-model request counts to a
single percentage-based "general" quota. Parse current_interval/weekly
_remaining_percent + status, close cycles for discontinued models, and
make the cycle overview show the active model (not the stale shared
pool). Widen MiniMax quota cards and drop the Duration/Total Delta
columns from its cycle overview.

Harden tests: isolate OPENCODE_HOME/XDG_DATA_HOME so credential
detection never reads the host's real OpenCode auth file.

Bump version to 2.12.0.