Implement SDK Class

[prosdev]

📋 Spec: specs/phase-1-core-sdk/tasks.md#task-6

Implement the main SDK class with capability composition and plugin registration system.

Files to create/modify:

• packages/core/src/sdk.ts
• packages/core/src/sdk.test.ts
• packages/core/src/index.ts (update exports)

Implementation:

• Event-based lifecycle (no separate Lifecycle class)
• Capability injection in use() method
• Method chaining support
• Delegate to capabilities (Emitter, Config)

Acceptance Criteria:

• Plugins can be registered via use()
• Multiple plugins work together
• Init/destroy lifecycle works
• Events flow correctly
• Test coverage > 90%
• Method chaining works (sdk.use().use().init())

Depends on: #2, #3, #4
Blocks: #7, #8

Priority: High