feat(opencode): add LiteLLM plugin integration

[RheagalFire]

Issue for this PR

Closes #29935

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds LiteLLM as a plugin. Sets LITELLM_BASE_URL (and optionally LITELLM_API_KEY) to connect. Models are synced from the proxy's /v1/models endpoint and enriched with per-model metadata from /model/info.

Files changed:

• src/plugin/litellm/litellm.ts: Plugin with provider.models hook (model sync) and auth hook (API key)
• src/plugin/litellm/models.ts: Model fetching from /v1/models + /model/info metadata enrichment
• src/plugin/index.ts: Registered LiteLLM in internal plugins
• src/provider/provider.ts: Removed built-in provider code, added generic plugin provider seeding
• packages/core/src/provider.ts: litellm provider ID
• sprite.svg / types.ts: Provider icon
• en.ts / settings-providers.tsx / dialog-select-provider.tsx: UI strings and notes

Env var	Description
LITELLM_BASE_URL	Base URL of the LiteLLM proxy (e.g. http://localhost:4000)
LITELLM_API_KEY	API key / master key for the proxy (optional if proxy has no auth)

Related: #29308, #6231, #14468

How did you verify your code works?

1. Started LiteLLM proxy with config (claude-sonnet-4-6 + claude-opus-4-6 via Azure AI Foundry)
2. Set LITELLM_BASE_URL=http://localhost:4000 env var
3. Ran TUI (bun dev), LiteLLM models auto-discovered from /v1/models and appear in model picker
4. Verified /model/info metadata enrichment (token limits, costs)
5. Sent messages with streaming, works
6. Switched between models (sonnet/opus), works
7. Full test suite passes (2979 pass, 0 fail)
8. Typecheck passes across all 21 packages

Screenshots / recordings

ceb2-3aa7-47fd-b86f-e27af97bfe24-2.mp4

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate/Related PRs Found:

1. PR feat(opencode): add LiteLLM provider with auto model discovery #14468 - feat(opencode): add LiteLLM provider with auto model discovery

   • feat(opencode): add LiteLLM provider with auto model discovery #14468
   • Why related: This appears to be an earlier or alternative implementation of LiteLLM provider integration with auto model discovery. Current PR feat(opencode): add LiteLLM plugin integration #29937 is likely updating/completing this feature (PR feat(opencode): add LiteLLM provider with auto model discovery #14468 is mentioned in the "Related" section).

2. PR fix: inject cache_control on content blocks for openai-compatible proxies to Anthropic backends (Bifrost, LiteLLM, Databricks) #25985 - fix: inject cache_control on content blocks for openai-compatible proxies to Anthropic backends (Bifrost, LiteLLM, Databricks)

   • fix: inject cache_control on content blocks for openai-compatible proxies to Anthropic backends (Bifrost, LiteLLM, Databricks) #25985
   • Why related: Involves LiteLLM proxy handling and OpenAI-compatible protocol features that may be relevant to the integration.

3. PR feat(opencode): add LLM provider fallback chain #26292 - feat(opencode): add LLM provider fallback chain

   • feat(opencode): add LLM provider fallback chain #26292
   • Why related: Provider integration feature that may have overlapping scope with adding a new gateway provider.

The most significant is PR #14468 - verify if it's the predecessor/duplicate of the current work or if it's a separate issue being addressed by PR #29937.

[RheagalFire]

cc @adamdotdevin would love your review on this one.