FR: Support .jj redirect file for workspace repo discovery

[Buttars]

Is your feature request related to a problem? Please describe.
When using jj workspaces to replicate a bare+worktree layout, there is no lightweight marker file in the workspace directory that indicates where the repo root lives. Tooling (editors, shell prompts, scripts) that look for a .jj directory to identify repo boundaries won't find one in the workspace, making the layout harder to discover and work with automatically.

Describe the solution you'd like
Support a .jj file (not directory) in a workspace directory that contains a path to the repo root, mirroring how Git resolves a gitdir file. When jj encounters a .jj file, it would read the path and use that as the repo root instead of searching parent directories.

Example layout:

my-project/
├── .repo/        ← jj repo root (.jj/ directory lives here)
├── main/
│   └── .jj       ← contains: ../.repo
└── feat-1/
    └── .jj       ← contains: ../.repo

Running jj status inside my-project/main/ would resolve the repo at my-project/.repo/ without needing to pass --repo explicitly.

Describe alternatives you've considered

• Passing --repo explicitly on every command — works but is not ergonomic and requires the user to always know the relative path to the repo root.
• Running commands from inside .repo/ — defeats the purpose of having separate workspace directories.
• jj's existing workspace linkage stored inside .repo/.jj/ — functional, but produces no discoverable artifact in the workspace itself.

Additional context
This mirrors the pattern many Git users rely on when using bare repos with worktrees, where a gitdir file (e.g. echo "gitdir: ../.repo" > .git) makes each worktree self-describing. Supporting the same convention in jj would make migration from that workflow seamless and enable the same class of tooling integrations.