[FEATURE]: Support custom branch names when creating workspaces (worktrees) #20950

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Current Behavior:
When creating a new workspace (git worktree) through OpenCode's UI or API (POST /experimental/worktree), the branch name is automatically generated as opencode/<slugified-name>. Users cannot specify a custom branch name like feature/my-branch or bugfix/issue-123.

Use Case:
In many development workflows, teams use specific branch naming conventions (e.g., feature/, bugfix/, hotfix/ prefixes). The current auto-generated opencode/<name> format doesn't align with these conventions, making it harder to:

• Follow team branch naming policies
• Integrate with CI/CD pipelines that expect certain branch patterns
• Organize branches logically in git history

Workaround (Temporary Solution):
Currently, users can manually create a worktree with a custom branch name and register it in OpenCode:

# 1. Manually create worktree with custom branch name
git worktree add -b feature/my-custom-branch /path/to/worktree

# 2. Register it in OpenCode's database by updating the project sandboxes field

However, this requires manual database manipulation and is not user-friendly.

Proposed Solution:
Extend the workspace creation API to accept an optional branch parameter:

POST /experimental/worktree
{
  "name": "my-workspace",
  "branch": "feature/my-custom-branch"
}

If branch is provided, use it directly (after validating it doesn't already exist). If not provided, fall back to the current auto-generated opencode/<name> format.

Additional Context:
Looking at the code in packages/opencode/src/worktree/index.ts, the CreateInput schema and makeWorktreeInfo function would need to be modified to accept and use a custom branch name.