feat(coding-agent): add pi.registerProvider() for dynamic provider registration

[dannote]

Problem

Extensions cannot dynamically configure model providers. The only way to add providers is through models.json, which doesn't support:

• Fetching config from external servers at runtime
• Conditional registration based on environment (e.g., local Ollama)
• Team/corporate provider distribution via extensions

Solution

Add pi.registerProvider(name, config) to ExtensionAPI for runtime provider registration.

Use cases

• Fetch provider config from corporate/team server dynamically
• Conditionally register local models (e.g., Ollama if running)
• Override baseUrl for existing providers (proxies, private endpoints)
• Project-specific model configurations

Example

// Fetch config from server
const config = await fetch('https://internal.company.com/ai-config').then(r => r.json());
pi.registerProvider('company-ai', config);

// Override existing provider's baseUrl  
pi.registerProvider('anthropic', {
  baseUrl: 'https://private-ai.company.com'
});

Changes

• model-registry.ts: Add registerProvider() method
• types.ts: Add ProviderConfig, ProviderModelConfig types, extend ExtensionAPI
• loader.ts: Pass ModelRegistry to extensions, implement API method
• sdk.ts: Thread modelRegistry through extension loading
• extensions.md: Document new API
• Tests for new functionality

[badlogic]

I like this! I'm in the middle of a big refactor, so it will take a while for me to get to this. Just a few initial thoughts:

• I think it would be nice, if registerProvider() could also optionally take an interface that would allow inclusion in the list of oauth providers. That way, people could implement their own /login. Needs a bit of plumbing and some thinking wrt API design of registerProvider() but should be totally possible.
• Shouldn't be part of this PR, but a possible future change: also allow registration of a provider API implementation, basically what's found in anthropic.ts and consorts. This is likely more complex, but would open up a lot of things wrt custom endpoints, support for providers not yet in core, etc.