Feat: Productization baseline and support-tier contract

[marco0560]

Summary

Make Codira usable as a product through init, doctor, isolated or one-command installation, support-tier documentation, demo repositories, and release smoke coverage.

Acceptance criteria

• init and doctor cover plugins, backend, model, and index health.
• One-command installation path is documented and smoke-tested.
• Tier A semantic, Tier B structural, and Tier C text support are explicit.
• Linux, macOS, and Windows release smoke checks exist.

Dependencies

• Feat: Ship MCP client presets and compatibility integration suite #65