refactor: modular provider plugin architecture (upstream parity)

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Problem

All 18 provider loaders live as inline async functions inside a single CUSTOM_LOADERS record in provider.ts (lines 84-595, ~511 lines). Adding or modifying a provider means editing this monolithic file, which:

• Increases merge conflict risk
• Makes individual providers hard to test in isolation
• Inflates cognitive load when working on a single provider
• Diverges from upstream's modular architecture

Current providers in CUSTOM_LOADERS

#	Provider	Lines	Complexity
1	anthropic	85-131	High — OAuth, subscription, API key, beta headers
2	cyberstrike	132-153	Medium — auto-filter paid models, public fallback
3	openai	154-162	Low
4	github-copilot	163-172	Low — responses/chat routing
5	github-copilot-enterprise	173-182	Low
6	azure	183-195	Low — responses/chat routing
7	azure-cognitive-services	196-211	Low
8	amazon-bedrock	212-371	Very high — AWS credential chain, cross-region, profiles
9	openrouter	372-382	Low — headers only
10	vercel	383-393	Low — headers only
11	google-vertex	394-418	Medium — ADC, project/location resolution
12	google-vertex-anthropic	419-446	Medium — regional endpoints
13	sap-ai-core	447-470	Medium — AICORE_SERVICE_KEY parsing
14	zenmux	471-481	Low — headers only
15	gitlab	482-523	Medium — OAuth/API, feature flags
16	cloudflare-workers-ai	524-546	Medium — account ID resolution
17	cloudflare-ai-gateway	547-584	Medium — unified API format
18	cerebras	585-595	Low — header only

Proposal

Refactor to per-provider files under src/provider/loaders/, mirroring upstream's modular plugin approach:

src/provider/
├── provider.ts          # core loading logic, imports loaders
├── loaders/
│   ├── index.ts         # re-exports all loaders as CUSTOM_LOADERS record
│   ├── anthropic.ts
│   ├── amazon-bedrock.ts
│   ├── google-vertex.ts
│   ├── google-vertex-anthropic.ts
│   ├── gitlab.ts
│   ├── cloudflare-workers-ai.ts
│   ├── cloudflare-ai-gateway.ts
│   ├── sap-ai-core.ts
│   ├── openrouter.ts    # (could group simple header-only loaders)
│   └── ...

Each loader file exports a single async function matching the current CustomLoader type signature. The loaders/index.ts assembles them into the CUSTOM_LOADERS record — zero change to the consumer API.

Suggested phases

1. Extract complex providers first — amazon-bedrock (160 lines), anthropic, google-vertex*, gitlab, cloudflare-*
2. Group simple header-only providers — openrouter, vercel, zenmux, cerebras could share a utility
3. Add per-provider tests — currently untestable in isolation
4. Align type with upstream's PluginV2.define() if applicable

Upstream reference

Upstream (anomalyco/opencode) uses a granular plugin architecture at packages/core/src/plugin/provider/ with PluginV2.define(), typed event hooks (catalog.transform, aisdk.sdk, aisdk.language), and Effect-based composition.

We don't need to adopt Effect or the full plugin system — the goal is file-level separation while keeping the existing CustomLoader interface.

Non-goals

• Changing the provider loading flow (6-step process stays)
• Adopting Effect or upstream's full plugin system
• Breaking the CustomLoader type contract