docs: define the persistent conversation and channel model

[ibourgeois]

Closes #102

Summary

• define conversations as durable workspace-scoped collaboration spaces rather than disposable transcript sessions
• capture the intended hierarchy of workspaces, conversations or channels, participants, and linked graph objects
• document the product assumption of one persistent space per participant set for users, models, and agents
• align the high-level v2 overview with the new persistent conversation model

Verification

• git diff --check

Notes

This is a product and architecture guidance PR, not an implementation PR. It is meant to guide the upcoming shell mockup, component, and feature-flag work without prematurely locking in storage or transport details.

[Copilot]

Pull request overview

Defines Katra v2’s intended “persistent conversation/channel” concept as a durable, workspace-scoped collaboration space (closer to Slack/Teams) and updates the v2 overview to reflect this navigation/product model.

Changes:

• Adds an explicit v2-overview UX bullet emphasizing durable workspace-scoped conversations with people, models, and agents as first-class participants.
• Documents the persistent conversation/channel hierarchy, supported interaction shapes, and shell implications in the v2 product/architecture principles doc.
• Adds a reference link to the tracking issue for the conversation/channel model definition work.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
docs/v2-overview.md	Updates the high-level v2 UX direction and links to issue #102 for the conversation/channel model.
docs/architecture/v2-product-and-architecture.md	Adds detailed guidance for the persistent conversation/channel model, assumptions, and what remains deferred.

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