feat(provider): add OpenCode Free -- keyless public-tier provider

[Wibias]

What

Adds OpenCode Free (opencode-free) as a new provider, giving users access to opencode.ai public bearer-token tier without any API key.

How it works

opencode.ai exposes a public desktop tier authenticated with Authorization: Bearer public and x-opencode-client: desktop -- the same credentials their own desktop app sends.

Infrastructure: staticHeaders on registry entries

Adds a staticHeaders field to ProviderRegistryEntry propagated as provider.headers. Also fixes merge order in createOpenAIChatAdapter so static headers apply first and a user-supplied apiKey can override them.

GUI: Free badge + optional key field

• Exposes keyOptional through DerivedProviderPreset and entryToPreset so /api/provider-presets surfaces it to the GUI picker
• AddProviderModal: green Free badge in picker list, no API key field by default (replaced with a green Free tier banner), collapsible Use your own key instead section for power users

Tests

9 new unit tests in tests/opencode-free-provider.test.ts: registry shape, staticHeaders propagation, providerConfigSeed, key-login map presence, featured list, no-key request headers, user-key override, note content, and deriveProviderPresets keyOptional exposure. All 9 tests pass.

Risk

Low. The staticHeaders mechanism is additive; no existing provider uses it. The public tier may be rate-limited or revoked by opencode.ai at any time.

[lidge-jun]

Merged into dev and released in v2.7.19 — thanks! The Zen endpoints checked out against OpenCode's official docs/source. Stacked fixes (3932acb): restored the pre-existing header precedence in openai-chat (registry staticHeaders still flow via derive), fixed the unused hints lint error that was failing CI on the branch, bounded the non-JSON error-body log inspection to an 8 KiB prefix (full response still reaches the client), and added a free-tier data-use warning to the provider note.