docs: add "Why pistachio?" section to README

[winebarrel]

Summary

• Add a Why pistachio? section near the top of the README (right after the intro and "Getting Started" link, before Installation).
• Frames the motivation against hand-written migration files and highlights four pillars: declarative editing, plain-SQL files with pista dump round-trip, safe-by-default drop suppression (--allow-drop / -- skipped:), and production-aware apply controls (CONCURRENTLY, --with-tx, --pre-sql / lock_timeout).

The framing was informed by how comparable declarative tools (sqldef, Atlas, Skeema) present their value, while emphasizing pistachio's differentiators (drop suppression visibility, online-apply controls, dump round-trip).

Test plan

• Render the README on GitHub and confirm the new section appears in the intended location with correct Markdown formatting.

https://claude.ai/code/session_01PpzYzcj7YYUVnieiUTCTVv

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.94%. Comparing base (e7e0f8d) to head (e0a6771).

Additional details and impacted files

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  Coverage   91.94%   91.94%           
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  Files          50       50           
  Lines        5425     5425           
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  Hits         4988     4988           
  Misses        230      230           
  Partials      207      207           

Flag	Coverage Δ
pg15	91.90% <ø> (ø)
pg16	91.90% <ø> (ø)
pg17	91.90% <ø> (ø)
pg18	91.94% <ø> (ø)

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[Copilot]

Pull request overview

Adds a new “Why pistachio?” section near the top of the README to better explain the tool’s motivation and differentiators (declarative workflow, plain-SQL schema source of truth with dump round-trip, safe-by-default drop suppression visibility, and production-aware apply controls).

Changes:

• Introduces a “Why pistachio?” section immediately after the intro / Getting Started link.
• Highlights four core value pillars (declarative editing, pista dump round-trip, opt-in destructive changes with visible -- skipped: output, and online/production apply controls like CONCURRENTLY and --with-tx).

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