refactor!: use abstract provider classes

[oritwoen]

I want provider integrations to be normal classes here, not factory-shaped objects. Registry now takes constructors for abstract Provider, while search, read and reachability stay separate capabilities so read-only provider doesn't need fake search(). This is breaking for custom registrations, no compatibility shim.

[cubic-dev-ai]

All reported issues were addressed across 32 files

Architecture diagram

sequenceDiagram
    participant Caller as Caller (CLI/Extension/AI)
    participant Registry as Registry
    participant Base as Provider (abstract)
    participant Concrete as Concrete Provider (e.g., Brave)
    participant Error as Error Handler

    Note over Caller,Registry: NEW: Registration via class constructors
    Concrete->>Registry: register(ProviderClass) (static providerName, defaultBaseURL)
    Registry-->>Concrete: stored

    Note over Caller,Base: NEW: Capability-aware creation
    Caller->>Registry: createSearchProvider(name, config)
    Registry->>Registry: lookup ProviderClass
    Registry->>Concrete: new ProviderClass(config)
    Concrete->>Base: super(config, ProviderClass)
    Base->>Base: validate baseURL (http/https) - throws InvalidProviderUrlError if not
    Base-->>Concrete: initialized
    Registry->>Registry: isSearchProvider(provider) check
    alt provider lacks search()
        Registry->>Error: SearchNotSupportedError
        Error-->>Caller: error handled in command (exit 1)
    else provider supports search
        Registry-->>Caller: SearchProvider instance
        Caller->>Concrete: search(query, options)
        Concrete-->>Caller: SearchResult[]
    end

    Note over Caller,Registry: NEW: Read capability via createReadProvider
    Caller->>Registry: createReadProvider(name, config)
    Registry->>Concrete: new ProviderClass(config)
    Registry->>Registry: isReadProvider(provider) check
    alt provider lacks read()
        Registry->>Error: ReadNotSupportedError
        Error-->>Caller: thrown from readUrl()
    else provider supports read
        Registry-->>Caller: ReadProvider instance
        Caller->>Concrete: read(url, options)
        Concrete-->>Caller: ReadResult
    end

    Note over Caller,Registry: Availability probe (resolve.ts)
    Caller->>Registry: create(name)
    Registry->>Concrete: new ProviderClass(config)
    Caller->>Concrete: isAvailable()? (only if isAvailabilityProvider)
    Concrete-->>Caller: boolean (reachable)

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<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

0 issues found across 5 files (changes from recent commits).

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Requires human review: Breaks public API: register and create signatures changed, custom provider integrations require update without compatibility shim. Needs human review to confirm migration approach.

Re-trigger cubic</sub>