[Requests and ideas] Folder/Collection projects don't support per-repo default branches

**Mood:** 😊
**Category:** Requests and ideas

Folder/Collection projects don't support per-repo default branches
Problem

When creating a new session in a collection project (a folder containing multiple git repositories), the app doesn't respect or allow configuration of default branches for each child repository.

Current Behavior

Individual repository projects have a default_branch field in the projects table
Collection projects have an empty default_branch field
The collection_members table (which tracks child repos) has no default_branch column
When creating a new worktree session for a collection, there's no way to specify which branch each child repo should branch from
The app appears to use the main checkout's current branch, not a configured default
Expected Behavior

Collections should support per-repository default branch configuration, so that when creating a new session:

repo-a branches from development
repo-b branches from main
repo-c branches from master
Each child repository in the collection should be able to have its own default branch configured.

Impact

This makes collection projects difficult to use in multi-repo workflows where different repositories follow different branching conventions (e.g., main vs development vs master).

---
| Field | Value |
| --- | --- |
| App version | 0.2.24 |
| OS | macOS 26.5.1 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Day 2 (Week 1) |

Start by tracing collection session creation and how the projects and collection_members tables supply branch information. Compare the existing project default_branch behavior with collection members, then verify that a new session can use a distinct configured branch for each child repository, including main, development, and master.

索引モデルが issue の本文から書いたものです。