docs: publish Unity 7 readiness plan

[dudetru25]

What changed

• replace retired dynamic Marketplace badges with a stable install badge linked to the existing Marketplace item
• refresh the root, extension, package, Zed, remote-workspace, agent, MCP, and experiment documentation
• add a canonical documentation index and Unity 7 readiness plan
• add dated Unity 7, Unity CLI/Pipeline, and official MCP assessments
• add evidence-gated WS1 through WS8 implementation plans
• distinguish declared, validated, shipped, planned, pending, watch, and historical status
• document the screenshot-copy feature and synchronize its implementation into the embedded sample package

Why

The public README badges used retired Shields endpoints. Unity 7/CoreCLR preparation was spread across untracked research and task notes, and some wording could be read as support before exact-version proof existed. The sample package also lacked the screenshot-copy implementation described by its mirrored README.

Impact

This keeps the existing VS Code Marketplace item. It does not create a new listing. Unity 2019.4 remains a declared package baseline, Unity 6000.3.9f1 on macOS is the local sample baseline, Windows proof remains pending, and Unity 7 remains planned and evidence-gated.

Validation

• independent Luna/max documentation and code review: PASS
• local Markdown links: PASS for all 34 updated documentation files
• mirrored package README and changelog checks: PASS
• sample/root screenshot capture source and meta comparison: PASS
• git diff --check: PASS
• npm run validate: PASS (205 runtime, 10 simplified-context, 10 remote-shell tests; vendor validation; zero npm audit findings)
• npx --no-install vsce package --no-dependencies: PASS

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