docs: reposition Heddle around the SDK

[roackb2]

Summary

• Reframe Heddle as an SDK for building agentic experiences inside products, with the local coding agent presented as the reference host and fastest evaluation path.
• Add progressive onboarding from running Heddle, to embedding the SDK, to owning models, tools, policy, persistence, UI, and transport.
• Document ownership boundaries, customization depth, core capabilities, and production posture in both English and Traditional Chinese.

Why

The root README was centered on the local coding-agent experience, which made Heddle's broader programmatic value difficult to discover. This rewrite makes the SDK the primary product story while preserving the CLI as a concrete way to learn the runtime.

Impact

• Product builders can identify the right SDK entry point and understand how far Heddle can be customized.
• CLI users keep a clear quick-start path and can graduate naturally into embedding Heddle.
• Documentation only; there are no runtime or API behavior changes.

Validation

• git diff --check
• Parsed both README files with Remark and GFM
• Verified all 92 local README links resolve