docs: explain Codex plugin workflow

[SetraTheXX]

What changed

• distinguish npm CLI installation from Codex plugin registration
• document local marketplace installation on Windows, macOS, and Linux
• explain the roadmap-driven CEWP product interaction and its current boundaries
• add a contract test that keeps the installation guidance present

Why

Users need a clear path from the npm package to the full Codex plugin and an honest explanation of how CEWP complements native goals and subagents.

Validation

• npm run test:plugin-package
• npm run test:pilot-release
• npm run test:contributor-surface
• npm run test:workflow-docs
• npm run check
• npm run pack:dry-run
• git diff --check

[Copilot]

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: ef23ecdfbb

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[chatgpt-codex-connector]

Align the install guide with the npm marketplace path

When users follow the linked installation documentation after this new global-package procedure, docs/install.md:69-75 still says the beta plugin must be installed from a source checkout and that npm distributes only the CLI, while docs/validation-status.md:12 still labels plugin distribution as source-checkout-only. This leaves the supported installation boundary contradictory and may cause npm users to clone the repository unnecessarily or distrust the new commands; update the canonical install and validation documents together with this claim.

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