docs: restructure README around reader personas

[jlouvel]

Related Issue

Closes #63

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What does this PR do?

Restructures the Polychro README.md around reader personas, mirroring the Ikanos README change (naftiko/ikanos#620) and based on the same user feedback: the README opened with dense, cross-domain jargon before the reader could tell whether the project was for them.

Changes:

• Adds a "Polychro in one sentence" jargon-light hook at the top.
• Adds a "Who are you?" section with three persona blocks in a "Who you are → What Polychro can do for you" shape:
  • 🤖 You're building AI agents or tools (MCP, in-process, agent-format output)
  • 🛡️ You run CI or a platform (GitHub Action, SARIF, one binary, governance rulesets)
  • 📐 You author specs or rules (Spectral rulesets, polyglot custom functions, validator SPI)
• Each persona links to the relevant guide to start with (MCP server / GitHub Action / Rulesets).
• Renames "What Polychro is" to "Under the hood" and keeps the technical tables, quick start, and comparison below as reference.

No feature claims are added or changed — this is purely a structure/onboarding improvement.

The companion docs changes (landing page + new who-is-it-for.md) are in the Shipyard repo.

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Tests

No tests — documentation-only change. Verified Markdown renders cleanly and the referenced guide links resolve.

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Checklist

• CI is green (build, tests, schema validation, security scans)
• Rebased on latest main
• Small and focused — one concern per PR
• Commit messages follow Conventional Commits

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Agent Context (optional)

agent_name: GitHub Copilot
tool: vscode
confidence: high
discovery_method: user_report

[eskenazit]

LGTM