docs: define the desktop MVP shell and navigation

[ibourgeois]

Closes #100

Summary

• define the first real Katra desktop shell as a branded workspace surface instead of a runtime diagnostic page
• capture the Codex-inspired but non-developer desktop direction and the Slack/Teams/WebEx-style navigation model
• document the MVP shell structure, conversation principles, brand requirements, and implementation guardrails for follow-up UI work
• align the high-level v2 overview with the new desktop shell direction

Verification

• git diff --check

Notes

This is intentionally a product and architecture guidance PR, not an implementation PR. It is meant to inform follow-up issues for the shell mockup, Pennant rollout, component system, conversation model, and runtime diagnostics relocation.

[Copilot]

Pull request overview

Updates the v2 documentation to define the intended “first real” Katra desktop shell as a branded workspace surface (not a developer/runtime diagnostic page), and aligns the v2 overview with that UX/navigation direction.

Changes:

• Expanded docs/v2-overview.md to explicitly call out the shift from the current developer-oriented landing page to a branded workspace shell, plus a summary of the intended desktop UX/navigation model.
• Added a new “Desktop MVP Shell Guidance” section to docs/architecture/v2-product-and-architecture.md, covering shell purpose, navigation principles, brand requirements, and implementation guardrails.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
docs/v2-overview.md	Adds concise status + UX-direction bullets and links shell definition work to issue #100.
docs/architecture/v2-product-and-architecture.md	Defines the desktop MVP shell guidance, including what to center/avoid and explicit brand constraints linking to brand docs/assets.

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