[integrations-posthog] Make PostHogClient resolvable through Croco DI with explicit configuration

[kang-heewon]

Priority: P1

Problem

PostHogClient is marked as a Croco component but its constructor requires an erased TypeScript interface and no configuration token or factory is exported. Normal Container resolution cannot provide its required configuration.

Evidence

packages/integrations-posthog/src/libs/PostHogClient.ts:10-17 declares @Component() with constructor(config: PostHogConfig); the package index and README expose direct construction but no DI configuration token or factory.

Desired outcome

The public component integration surface works through Croco DI with a validated configuration contract.

Implementation path

1. Export a configuration token and provider/factory.
2. Validate and register configuration before resolving the client.
3. Inject the runtime value through the token.
4. Add container resolution and missing-configuration tests.

Acceptance criteria

• Container.get(PostHogClient) resolves after documented setup.
• Missing or invalid configuration yields a stable diagnostic.
• Direct construction remains intentional or is explicitly deprecated.

Validation

• pnpm --filter @croco/integrations-posthog test
• pnpm --filter @croco/integrations-posthog typecheck

Scope boundaries

Does not add new PostHog event features.