[Feature]: Project-scoped scratchpad shared across threads

[burnmandont]

Problem or use case

I often need to keep short-lived project context—commands to run, temporary TODOs, review reminders, and handoff notes—where it is visible from every thread in a project.

Thread drafts are tied to one conversation, while files such as AGENTS.md are durable repository instructions and may be committed. An external notes app loses the project association and is unavailable from connected T3 Code clients.

Smallest useful scope

Would a small project-scoped scratchpad be in scope?

• One plain-text note per project
• Accessible from the chat header
• Shared by every thread in that project
• Persisted by the project's T3 server
• Synchronized to connected web, desktop, and mobile clients
• Autosaved with bounded update frequency and an explicit size limit
• Last-write-wins if two clients edit it concurrently

The note would be user-facing only. It would not be included in agent prompts.

Non-goals

• Rich text or formatting controls
• Multiple notes
• Note history or collaborative cursor merging
• Agent read/write access
• Automatic prompt injection
• Global or per-thread notes

Related proposal

#2396 proposes session-scoped notes, including an agent-shared region injected into prompts. It also mentions project-scoped private notes as a possible sibling proposal. This idea is that narrower sibling: project-scoped, private to the T3 UI, and never sent to the model.

Implementation direction

A focused implementation could reuse the existing project metadata command/event/projection path and add one field to the project projection. We have a working version in our fork, but would reimplement it cleanly against current main with focused persistence, synchronization, web, and mobile tests.

Is this feature in scope, and would maintainers consider a focused PR? If so, is project metadata the preferred persistence model?