Allow opening one project across multiple directories/worktrees without requiring local splitting workarounds

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

Problem

Currently, when a single project needs to be opened or worked on from multiple directories, users may need local workaround flows such as splitting-opencode-projects. This adds friction and makes multi-directory workflows feel like something users have to manually adapt around instead of something opencode supports directly.

Use case

Some projects naturally span multiple local directories, for example:

• monorepos split across several working directories
• generated/distribution mirrors next to source repositories
• multiple worktrees for parallel feature work
• related project folders that should share one opencode project/session context

In these cases, it would be useful for opencode to treat several directories as belonging to the same project identity.

Requested behavior

Allow a project to be opened from multiple configured directories without requiring a local skill/workaround.

Possible approaches:

• Support multiple root paths for a single project
• Let users define project aliases that map several directories to the same project
• Allow session/project identity to be configured independently from the current working directory
• Provide first-class support for multi-directory or multi-worktree project contexts

Expected benefit

This would make opencode smoother for users working with monorepos, mirrors, generated repos, and parallel worktrees. It would also reduce the need for custom local skills that compensate for project-directory limitations.