[FEATURE] Desktop app: make a project a saved multi-folder config with its own new-session plus

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

In the Claude Code desktop app, the sessions sidebar has two kinds of section that look identical but behave differently.

• Folder sections are generated automatically from a session's working directory, labeled with the folder name, and get a + button to start a new session in that folder.
• Custom groups are labels I create. They can hold sessions from many directories, and they get no + because a group has no single directory to start in.

So I can have a plus, or I can manage my projects, but not both. A project of mine that spans two repositories (a common case) can only be set up as a multi-folder session by hand at creation time, and in a local desktop session there is no UI control to add a folder afterward. Once created, that configuration cannot be reused for the next session on the same project; every new session starts from a single folder and I re-add the rest.

Proposed Solution

Make a project a saved configuration rather than a folder: a name, one or more folders, and whatever else a session needs at creation. Then:

• The + on a project creates a new session from that configuration, all folders included.
• A session belongs to the project that created it, so grouping falls out for free and the two sidebar section types collapse into one.
• Editing the project's folder list updates what future sessions get, and ideally what an open session has access to.

Alternative Solutions

• Start the session from a folder section (to get the plus) and then file it into a custom group by hand. Works, but the multi-folder part still has to be redone every time.
• Ask Claude to request access to the extra folder mid-session. This grants access, but the header chip does not reflect it and nothing is saved for next time.
• The CLI's /add-dir covers the mid-session case for terminal users, but has no visible equivalent in the local desktop session UI.

Priority

Medium - Would be very helpful

Feature Category

Other

Use Case Example

1. I maintain a framework in one repository and a project that uses it in another.
2. Nearly every session needs both folders.
3. Today I either create a multi-folder session by hand each time, or start from one folder and lose access to the other.
4. With a project config, I would press + on "Framework work" and get both folders every time, and the session would land in the right sidebar group automatically.

Additional Context

Observed on macOS desktop app. Two sections named "Megacosm" (a custom group) and "megacosm" (a folder section) sit side by side in my sidebar with different affordances, which is how I noticed the split.