docs: add "Why This Fork?" section to README (LAC-1469)

[lacymorrow]

Summary

• Adds a "Why This Fork?" section to the lash README explaining the motivation for forking and what features we maintain on top of upstream
• Follows the same pattern used in the Paperclip fork (docs: explain why this repo is a fork (LAC-1463) paperclip#20)
• Documents lash-specific features: shell mode, cwd tracking, natural language detection, double border UI, tab completion, SDK events, and independent versioning
• Rebased onto latest lash branch to resolve merge conflicts from upstream sync

Paperclip Issue

LAC-1469

Test Plan

• README renders correctly on GitHub
• Section appears before installation instructions
• No conflicts with updated base branch

[gemini-code-assist]

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